There is much confusion among Catholics less than 55 years old on the nature of grace. This is owing to bad catechesis and the Protestant mindset which crept into the Church in the 1970s. I am reposting this and one can go back and read the first four posts in this mini-series.
Christian Perfection and Contemplation According to St. Thomas Aquinas
and St. John of the Cross
by Fr. Reginald Garrigou-Lagrange, O.P. (What would I do without this great man?)
Copyright 1937, Herder Book Co., 1937
Imprimatur and Nihil Obstat, 1937
The Practical Consequences of the
Doctrine of St. Thomas on Grace
St. Thomas, following St. Augustine and opposing Pelagian or semi-Pelagian naturalism, grasped the depth and the height of our Lord's words: "Without Me you can do nothing,"  and of St. Paul's words: "For it is God Who worketh in you, both to will and to accomplish, according to His good will."  "For who distinguisheth thee? Or what hast thou that thou hast not received?"  In the work of salvation we cannot distinguish any part that is exclusively ours; all comes from God, even our free co-operation, which efficacious grace gently and mightily stirs up in us and confirms.
This grace, which is always followed by its effect, is refused to us, as we said, only if we resist the Divine, auxilium praeveniens, sufficient grace, in which the efficacious help is already offered us, as fruit is in the flower. If we destroy the flower, we shall never see the fruit, which the influence of the sun and of the nourishment of the earth would have produced. Now man is sufficient to himself to fall; drawn from nothingness, he is by nature defectible. He is sufficiently assisted by God so that he falls only through his own fault, which thus deprives him of a new help. This is the great mystery of grace. We have elsewhere explained what St. Thomas and his best disciples teach about this mystery. 
With him and St. Augustine we must submit our intelligence before this Divine obscurity, and as Bossuet says, "confess these two graces (sufficient and efficacious), one of which leaves the will without excuse before God, and the other does not permit the will to glory in itself."  Is this not in conformity with what our conscience tells us? According to this doctrine, all that is good in us, naturally or supernaturally, has its origin in the Author of all good. Sin alone cannot come from Him, and the Lord allows it to happen only because He is sufficiently powerful and good to draw from it a greater good, the manifestation of His mercy or justice.
All grace is from God, all....and our free will either hinders or encourages holiness.
This teaching of the great doctors of grace lifts our mind to a lofty contemplation of God's action in the innermost depths of our heart. To prove this, we have only to demonstrate that this doctrine should lead those who understand it well to profound humility, to almost continual interior prayer, to the perfection of the theological virtues and of the corresponding gifts of the Holy Ghost. Besides, we find it in the writings of all the great masters of the spiritual life. Considering the importance and the difficulty of the problem, we shall affirm nothing in this article except according to the very words of Scripture, as the greatest doctors explain them.
This doctrine leads first of all to profound humility. According to this doctrine man has as his own, as something coming exclusively from himself, only his sin, as the Council of Orange declared. He never performs any natural good act without the natural aid of God, or any supernatural good act without a grace which solicits or attracts him, and also efficaciously moves him to the salutary act. As St. Paul says: "Not that we are sufficient to think anything of ourselves as of ourselves; but our sufficiency is from God."  Even holy souls that have reached a high degree of charity are always in need of an actual grace in order to merit, to advance, to avoid sin, and to persevere in goodness.
At every state, we need grace.
 They should say: "For the thoughts of mortal man are fearful, and our counsels uncertain,"  "Thy will be done on earth as it is in Heaven . . . and lead us not into temptation." After striving greatly, they should admit: "We are unprofitable servants,"  for the Lord might have chosen others who would have served Him much better. In all truth we should say, according to the teaching of St. Thomas, that there is no sin committed by another man which I might not commit in the same circumstances by reason of the infirmity of my free will, and of my own weakness (the Apostle Peter denied his Master three times). And if actually I have not fallen, if I have persevered, this is doubtless because I worked and struggled, but without Divine grace I should have done nothing. [ll] "Not to us, O Lord, not to us; but to Thy name give glory";  "as the potter's clay is in his hand, to fashion and order it . . . so man is in the hand of Him that made him."  "Thy hands have made me and formed me"; [l4] "Thou hast redeemed us to God, in Thy blood."  "If I have not perished, it is because of Thy mercy."  "Into Thy hands I commend my spirit."  "This," says St. Augustine, "is what must be believed and said in all piety and truth, so that our confession may be humble and suppliant, and that all may be attributed to God."  Such is true humility. "Or what hast thou that thou hast not received? And if thou hast received, why dost thou glory, as if thou hadst not received it?" 
The Saints, considering their own failures, say to themselves that if such and such a criminal had received all the graces the Lord bestowed on them, he would perhaps have been less unfaithful than they. The sight of the gratuity of the Divine predilections confirms them in humility. They recall our Lord's words: "You have not chosen Me: but I have chosen you."
This doctrine leads also to continual intimate prayer, to profound thanksgiving, to the prayer of contemplation.
It leads to intimate prayer; for this is a very secret grace that must be asked. We must ask not only the grace which solicits and excites the soul to good but also that grace which makes us will it, which makes us persevere, which reaches the depths of our heart and of our free will; that grace which moves us in these depths, so that we may be delivered from the concupiscence of the flesh and the eyes, and from the pride of life. God alone saves and snatches us from these enemies of our salvation. At the same time He does not wound our liberty, but establishes it by delivering us from the captivity of these things of earth.
Thus Scripture teaches us to pray: "Have pity on me, O Lord, according to Thine infinite mercy. Be propitious to a sinner. Help my unbelief. Create a clean heart in me, and renew a right spirit within me. Convert me, O Lord, make me return to Thee, and I shall return.  Thy will be done on earth as it is in Heaven." Give me Thy sweet and mighty grace in order that I may truly accomplish Thy holy will. As St. Augustine says: "Lord, give what Thou dost command, and command what Thou pleasest."
Thus again the Church prays in the Missal: "Lord, direct toward Thyself our rebellious wills; grant that unbelievers, who are unwilling to believe, may have a will to believe. Apply our hearts to good works. Give us good will. Convert us and draw us strongly to Thyself. Take from us our heart of stone and give us a heart of flesh, a docile and pure heart. Change our wills and incline them toward what is good." 
Such is the holy confidence of the prayer of the Church because she is sure that God is not powerless to convert the most hardened sinners. What should a priest do who cannot succeed in converting a dying sinner? Persuaded that God can convert this guilty will, above all the priest will pray. If, on the contrary, he imagines that God holds this will only from without, by circumstances, good thoughts, good inspirations, which remain external to the consent to salutary goodness, will not the priest himself delay too long in the use of superficial means? Will his prayer possess that holy boldness which we admire in the Saints, and which rests on their faith in the potent efficacy of grace?
I love the term here, "holy boldness" which is missing in most Catholics, who only want to be nice.
Likewise prayer should be, in a sense, continual, since our soul needs a new, actual, efficacious grace for every salutary act, for each new merit. With this in mind, we clearly see the profound meaning of our Lord's words: "We ought always to pray, and not to faint."  This truth is fully realized only in the mystical life, in which prayer truly becomes, as the fathers say, "the breath of the soul," which hardly ceases any more than that of the body. The soul constantly desires grace, which is like a vivifying breath renewing it and making it produce constantly new acts of love of God. Such ought to be the prayer of petition. And we ought also to thank God for all our good actions, since without Him we could have done nothing. This is what makes St. Paul say: "Pray without ceasing. In all things give thanks; for this is the will of God in Christ Jesus concerning you all."  "Speaking to yourselves in psalms and hymns and spiritual canticles, singing and making melody in your hearts to the Lord; giving thanks always for all things, in the name of our Lord Jesus Christ, to God and the Father." 
This doctrine of the intrinsic efficacy of grace leads also directly to the prayer of contemplation, which considers chiefly the profound action of God in us to mortify and to vivify, and which is expressed by the fiat of perfect abandonment. In contemplation we see realized in the intimate depths of souls the words of Scripture: "Thou are great, O Lord, forever. . . . For Thou scourgest, and Thou savest: Thou leadest down to hell, and bringest up again."  "Thy word, O Lord, which healeth all things."  To utter a perfect fiat to this intense and hidden work of grace in us, even when it crucifies and seems to destroy all, is the most secret but also the most fruitful co-operation in God's greatest work. It is the prayer of Jesus in Gethsemane and that of the Blessed Virgin at the foot of the Cross.
If we are not crucified with Christ we shall not be perfected. Sometimes, it does seem like God is destroying all. A spouse dies, a child walks away from God and the Church, one loses a job, one gets cancer. Yet, these are the raw material for holiness.
Lastly, this doctrine reminds us that even for prayer efficacious grace is necessary. "Likewise the Spirit also helpeth our infirmity. For we know not what we should pray for as we ought; but the Spirit Himself asketh for us with unspeakable groanings. And He that searcheth the hearts, knoweth what the Spirit desireth; because He asketh for the Saints according to God." 
I know this is true. I cannot pray, meditate, or read Scripture without the grace of God. Now, we begin to understand those in the Unitive State, that last level of perfection on earth.
This mystery is verified especially in the mystical union, often obscure and painful, in which the soul learns by experience what great need we have of grace in order to pray, as also to do good. But, says St. John of the Cross,  souls that have reached a certain degree of union "obtain from God all that they feel inspired to ask of Him, according to the words of David, 'Delight in the Lord, and He will give thee the request of thy heart' " (Ps. 36:4). Moreover, every humble, confident, persevering prayer by which we ask what is necessary or useful for our salvation is infallibly efficacious, because our Lord uttered such a promise and because God Himself caused this petition to well up in our hearts. Resolved from all eternity to grant us His benefactions, He leads us to ask them of Him. 
The life of the virtues follows a life of grace, the sharing of divine life.
This doctrine of the powerful efficacy of grace leads finally to great heights in the practice of the theological virtues. This it does because it is intimately bound up with the sublime mystery of predestination, the grandeur of which it fully preserves. St. Paul, in the Epistle to the Romans, tells us: "And we know that to them that love God, all things work together unto good, to such as, according to His purpose, are called to be Saints. For whom He foreknew, He also predestinated to be made conformable to the image of His Son; that He might be the first-born amongst many brethren. And whom He predestinated, them He also called. And whom He called, them He also justified. And whom He justified, them He also glorified. What shall we then say to these things? If God be for us, who is against us?"  St. Paul teaches the same doctrine in the Epistle to the Ephesians. 
1. John 15:5.
2. Phil. 2:13.
3. See 1 Cor. 4:7.
4. God, His Existence and His Nature, II, 365 ff. It is not necessary that our failure precede the refusal of efficacious grace, in priority of time; priority of nature is sufficient, in the order of material causality, according to the principle of the mutual relations of causes explained by St. Thomas (Ia IIae, q.113, a.8 ad ium; cf. Ia IIae, q. l09, a. l, a. 8, 9, 10). It is God Who anticipates us by His grace when He justifies us, and it is we who are the first to abandon Him when we lose Divine grace: "God will not desert the justified, unless He is first deserted by them." Council of Trent, Sess. VI, chap. 2.
5. Bossuet, (Euvres complètes, 1845, I, 643. Cf. the general index of Bossuet's works for references to "grace" (resistance to grace). See particularly Defense de la tradition, Bk. XI, chaps. 19-27: Demonstration of the efficacy of grace by the permission of sins into which God allows the just to fall in order to humble them. Permission of the triple denial of St. Peter: "Peter was justly punished for his presumption by the withdrawal of an efficacious help which would have effectively hindered his denial." Bossuet shows that such is the doctrine not only of St. Augustine but of St. John Chrysostom, of Origen, of St. Gregory the Great, and of St. John Damascene, since they say that Peter was deprived of help, a statement which cannot apply to sufficient grace, for without this grace he would have been utterly powerless to avoid the sin. The statement applies to an efficacious help which would have made him effectively avoid this fall. From all of which we see that sufficient grace indeed leaves our will without excuse before God, and that the efficacious grace which St. Peter received later does not permit us to glory in ourselves.
6. Canon 22: "No one has anything of his own except his deceitfulness and his sin." Denzinger. no. 195.
7. See 2 Cor. 3:5.
8. Cf. Ia IIae. q. 109. a. 2. 8, 9, 10.
9. Wis. 9:14.
10. Luke 17:10.
11. Cf. Del Prado. O.P., De gratia, III, 151.
12. Ps. 113:1.
13. Eccles. 33:18; Jer. 18:6.
14. Ps. 118:73.
15. Apoc. 5:9.
16. Lam. 3:22.
17. Ps. 30:6; Luke 23:46.
18. De dono perseverantiae, chap. 13.
19. See 1 Cor. 4:7.
20. Lam. 5:21.
21. On these prayers of the Church, cf. St. Augustine, Epist. ad Vital., 217 (al. 107), and Bossuet, Defense de la tradition, Bk. X, chap. 10.
22. Luke 18:1.
23. See 1 Thess. 5:17-18.
24. Ephes. 5:19-20.
25. Tob. 13:2.
26. Wis. 16:12.
27. Rom. 8:25-27.
28. The Dark Night of the Soul, Bk. II, chap. 20.
29. Cf. IIa IIae. q. 83, a. 2; St. Augustine. Enchirid., chap. 32; Bossuet. Defense de la tradition, Bk. XII, chap. 38.
30. Rom. 8:28-31.
31. St. Paul also says in the Epistle to the Ephesians. 1:3-6. 11-12: "Blessed be the God and Father of our Lord Jesus Christ, Who hath blessed us with spiritual blessings, in heavenly places, in Christ: as He chose us in Him before the foundation of the world, that we should be holy and unspotted in His sight in charity. Who hath predestinated us unto the adoption of children through Jesus Christ unto Himself: according to the purpose of His will: unto the praise of the glory of His grace, in which He hath graced us in His beloved Son. . . . In whom we also are called by lot, being predestinated according to the purpose of Him Who worketh all things according to the counsel of His will. That we may be unto the praise of His glory, we who before hoped in Christ."
32. De praedestinatione sanctorum, chaps. 3, 6-11, 14, 15, 17; De dona perseverantiae, chaps. 1, 6, 7, 12, 16-20, 23; De correptione et gratia, chaps. 9, 12, 13, 14. See also on these texts, Del Prado, De gratia et Libero arbitrio, III, 555-564; II, 67-81, 259; and Bossuet, Defense de la tradition, Bk. XII, chaps. 13-20.
33. In Ep. ad Rom. 8:28; In Ep. ad Ephes., I, no. 5; Ia, q.23.
34. Cf 2 Pet. 3:9.