Garrigou-Lagrange has another book in which he looks at the Trinity. Now, the Mystery of the Triune God separates us from many other "faiths".
Unity is one of the Attributes of God. Here is a long selection from the book, The Trinity and God the Creator, and then a comment in blue.
First Article: Whether There Is A Trinity In God
The difficulty arises from the fact that everything that is triune is threefold, whereas God is not threefold since He possesses the greatest unity. Nevertheless the reply is in the affirmative as an article of faith. In the Athanasian Creed we read, "The Unity is to be worshiped in Trinity and the Trinity in Unity."[327]
Theology offers the following explanation. In God there is a transcendental plurality of persons. The term "Trinity" according to revelation limits this plurality to the three persons. Therefore the term Trinity can rightly be used.
Reply to the first objection. Etymologically the term "Trinity" seems to signify the unity of three, but in a special way it signifies the transcendental number of persons of one essence. Thus we cannot say that the Father is the Trinity. The term "Trinity" signifies at the same time the number of persons and the unity of the essence.
Reply to the second objection. St. John declared, "And these three are one" (I John 5:7). Hence we have the name "Trinity."
Reply to the third objection. Nevertheless in God there is no triplicity because triplicity denotes a proportion of inequality as do duplicity and quadruplicity. Thus we cannot say that God is threefold. That which is threefold has in a sense been tripled, as, for instance, a triple crown signifies the union of three crowns.
If God were said to be threefold, the three persons together would be more than one alone, and one person would not have infinite perfection. But we can say that the persons are threefold and the processions are twofold, because by adding person and procession we exclude sufficiently the multiplicity of nature.
Reply to the fourth objection. Unity in Trinity signifies that there is one nature in three persons, and Trinity in unity signifies three persons in one nature.
Reply to the fifth objection. We cannot say that the Trinity is threefold for this would mean that there were three supposita of the Trinity, whereas there are only three supposita of the Deity.
First corollary. From the foregoing the Thomists, especially Gonet, conclude that those things that belong to the persons by reason of the essence alone are predicated only singly. Those things, however, that belong to the persons by reason of the persons alone are predicated only in the plural. Those things that belong to the persons by reason of the essence and the relations are predicated both in the singular and in the plural.
The reason for this rule is that in God all things are one and the same except where there is the opposition of relation; only the relations are multiplied in God, the essence is not. This was defined by the Council of Toledo: "Number is discovered in the relation of the persons; but we find nothing that is numbered in the substance of the divinity. Thus number is indicated only in this, that they are mutually related; and they lack number in this, that they are in themselves."[328]
From this rule it follows that it is correct to say that there are three persons or three hypostases in God but not three individuals because the nature is multiplied in individuals. In its formal signification person denotes personality; in its material signification it denotes nature. On the other hand, the individual in its formal signification denotes nature; in its material signification it denotes personality.
This Truth to me is most awesome. God has "personality" and that is what distinguishes this definition of the Trinity from "individuality". That there are Three Persons in One God of one substance seems clear in the above clarification. Garrigou-Lagrange goes on to say that "...the Fourth Lateran Council, which said that the Father, the Son, and the Holy Ghost are "co-omnipotent and co-eternal, one principle of all things."[350]"
These Three Persons are One, but are knowable because of certain "notions" given through Revelation. These notions are defined as a "notion is that which is the proper reason for knowing a divine person."
Theology and philosophy, that is reason, can lead us to some understandings through the notions, but we need Revelation to help us know the Trinity.
Of course, Christ told us that He revealed the Father and that the Holy Spirit would come after His Ascension. That Christ came to reveal the Trinity is clear from the Annunciation. Christ became Incarnate by the power of the Holy Spirit and through the Fatherhood of God.
Mary was one of the first to have the Trinity revealed to her, and the only human to have experienced the Trinity as Three Persons intimately.
Personality indicates that God, like us made in His image and likeness, have knowledge of self. God reveals Himself to us as One, but in notions, which indicate relationship.
Here is the Dominican again.
Thus we have paternity, filiation, common active spiration, passive spiration, to which we add innascibility, because the person of the Father is known not only by paternity but also by the fact that He is from no one and that He is the principle without a principle. This notion is in conformity with the dignity of the Father, ....[370]
We may come to understand the Unity of God through Creation, through relationship with Him. His Unity is that there is only One God, One Infinite Being, as explained by the Church.
Here is Garrigou-Lagrange again.
The perfect intellect, however assimilates its Word most perfectly, not only intentionally, but really in nature and in a nature that is numerically one, so that the divine Word is not accidental but substantial, at the same time living and understanding, because in God being and understanding and being understood are the same. Revelation affirms that this substantial Word is the person of the Son of God. This is true generation, which primarily deserves the name generation; other kinds of generation are generation by participation and secondarily, although they are prior in our knowledge. Therefore St. Paul said," or this cause I bow my knees to the Father of our Lord Jesus Christ, of whom all paternity in heaven and earth is named" (Eph. 3:14 f.).[170]
My response to this small bit of explanation and guidance is pure awe and wonder. Actively contemplating on the Unity of God brings one closer to Him in this Oneness. Such is the grace He wants to give us.
I can only dip into the great complexity of the Attributes of God, but this little series may whet your appetite to study more.
To be continued.... and find more here https://www.ewtn.com/library/THEOLOGY/TRINITY.HTM#05