Here is the great saint on this point. God is not our debtor at all, but we give glory to him through the use of our own free will in obtaining from God that which He has.
Hence man's merit with God only exists on the presupposition of the Divine ordination, so that man obtains from God, as a reward of his operation, what God gave him the power of operation for, even as natural things by their proper movements and operations obtain that to which they were ordained by God; differently, indeed, since the rational creature moves itself to act by its free-will, hence its action has the character of merit, which is not so in other creatures.
Reply to Objection 1. Man merits, inasmuch as he does what he ought, by his free-will; otherwise the act of justice whereby anyone discharges a debt would not be meritorious.
Reply to Objection 2. God seeks from our goods not profit, but glory, i.e. the manifestation of His goodness; even as He seeks it also in His own works. Now nothing accrues to Him, but only to ourselves, by our worship of Him. Hence we merit from God, not that by our works anything accrues to Him, but inasmuch as we work for His glory.
Reply to Objection 3. Since our action has the character of merit, only on the presupposition of the Divine ordination, it does not follow that God is made our debtor simply, but His own, inasmuch as it is right that His will should be carried out.
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The Holy Spirit moves us to do acts of charity which gain merit, such as loving those who hate us and doing good for those who cannot pay us back, such as the poor.
Suffering patiently and with peace brings merit.
Man's meritorious work may be considered in two ways: first, as it proceeds from free-will; secondly, as it proceeds from the grace of the Holy Ghost. If it is considered as regards the substance of the work, and inasmuch as it springs from the free-will, there can be no condignity because of the very great inequality. But there is congruity, on account of an equality of proportion: for it would seem congruous that, if a man does what he can, God should reward him according to the excellence of his power.
If, however, we speak of a meritorious work, inasmuch as it proceeds from the grace of the Holy Ghost moving us to life everlasting, it is meritorious of life everlasting condignly. For thus the value of its merit depends upon the power of the Holy Ghost moving us to life everlasting according to John 4:14: "Shall become in him a fount of water springing up into life everlasting." And the worth of the work depends on the dignity of grace, whereby a man, being made a partaker of the Divine Nature, is adopted as a son of God, to whom the inheritance is due by right of adoption, according to Romans 8:17: "If sons, heirs also."
Reply to Objection 1. The Apostle is speaking of the substance of these sufferings.
Reply to Objection 2. This saying is to be understood of the first cause of our reaching everlasting life, viz. God's mercy. But our merit is a subsequent cause.
Reply to Objection 3. The grace of the Holy Ghost which we have at present, although unequal to glory in act, is equal to it virtually as the seed of a tree, wherein the whole tree is virtually. So likewise by grace of the Holy Ghost dwells in man; and He is a sufficient cause of life everlasting; hence, 2 Corinthians 1:22, He is called the "pledge" of our inheritance.
As adopted sons and daughters through baptism, we can do actions or pray to gain merit...
to be continued...