Sanctifying grace, which makes us pleasing to God, is not a mere extrinsic denomination, as when we say that we are seen or loved by human persons, or that a poor infant is adopted by a rich man. Grace is something real and intrinsic in our soul: "He hath given us most great and precious promises that by them you may be made partakers of the divine nature."  Whereas human love, as that of the rich man adopting a child, is given to what already exists, divine love creates something to be loved. Divine love is not sterile, and not merely affective, but effective and efficacious, creating, not presupposing, the good it loves. God cannot love a man without producing in that man a good, be it in the natural order, as when he gives him existence, life, and intelligence, or in the supernatural order, as when He makes man His adopted child, His friend, to prepare him for a blessedness wholly supernatural, wherein He gives Himself to man eternally. God's love, says St. Thomas,  creates goodness in creatures. Uncreated love does not presuppose, but creates, our lovableness in His eyes.
Thus St. Thomas excludes in advance the error of Luther, who says that man is justified solely by the extrinsic imputation to him of Christ's merits, without grace and charity being poured into his heart. This view is manifestly contrary to Scripture, which teaches that grace and charity were given to us by the Holy Ghost. .
Sanctifying grace, to proceed, is a permanent quality of the soul. It is the living water, springing up into eternal life.  It is "the seed of God,"  which tradition calls "the seed of glory."  St. Thomas  formulates a precise doctrine, which found ever wider acceptance and final approval in the Council of Trent.  We cannot hold, he says, that God provides less generously in the supernatural order than He does in the natural order. Since in the natural order He gives nature as radical, principle and the faculties as proximate principles of our natural operations, we may expect that He will give us grace as radical principle of our supernatural operations. Thus sanctifying graces becomes "a second nature," which enables us to connaturally know and love God in a higher order than that of our natural faculties.
This participation in the divine nature is indeed formal and physical, but only analogical.  Human words, even inspired words, far from being exaggerations, can express supernatural truths only by understatement. As the divine nature is the principle by which God knows and loves Himself, without medium or interruption, so sanctifying grace is the radical principle which disposes us to see God without medium, to love Him eternally without interruption, to do all things for His sake. That is the meaning of "participation in the divine nature." This participation is not a mere moral quality, a mere imitation of God's goodness. It is a real and physical participation, spiritual and supernatural, because it is the root principle of acts which are themselves really, physically, essentially supernatural. Human adoption gives to the child the moral right to an inheritance. Divine adoption creates in the soul a real and physical claim to divine inheritance.
Sanctifying grace, then, is a participation, not, like actual grace, virtual and transient, but formal and permanent. Still this participation is, not univocal, but analogical, because the divine nature is independent and infinite, whereas grace is essentially finite and dependent on God. Further, grace is an accident, not a substance, and the utmost knowledge it can give us of God is only intuitive, never absolutely comprehensive. Nevertheless this participation, though it is analogical, is still a participation in the deity as deity, since it is the source of the light of glory which enables us to see God as He is in Himself, the deity as deity. Now the deity as deity, though it pre-contains formally all perfections, being, life, intelligence, which it can communicate to creatures, still transcends infinitely all these perfections.  The stone, by participating in being, has an analogical resemblance to God as being. The plant, participating in life, has an analogical resemblance to God as living. Our soul, participating in intelligence, has an analogical resemblance to God as intelligent. But sanctifying grace alone is a participation in the deity as deity, a participation which is naturally impossible and hence naturally unknowable. Only the obscure light of infused faith here below, and only the light of glory there above, can let us see the deity as deity, God as He is in Himself.
We are here in a world of truth far beyond the reach of reason. Hence, first, the adversaries of the faith can never prove that sanctifying grace is impossible. But, secondly, neither can its possibility be rigorously demonstrated by reason. What, then, of the arguments we have just been proposing? They are arguments of appropriateness, profound indeed and inexhaustible, but since they move in an order beyond reason and philosophy, they can never be apodictically demonstrative. Both the intrinsic possibility of grace and its existence are affirmed with certitude, not by reason, but by faith alone. .
Grace, we must insist, is by its very nature absolutely supernatural. Angelic nature, since it far transcends human nature, is relatively supernatural, not essentially. Miracles are indeed absolutely supernatural, but only in the mode of their production, not in the effect they produce. The life restored miraculously to a corpse is in itself a natural life, not a supernatural life. But grace is absolutely supernatural, not in the mode of production merely, but in its very essence. Hence the remark of St. Thomas:  The grace even of one man is a greater good then the whole universe of nature. Only those who enjoy the beatific vision can fully know the value of grace, the source and root of their glory.  Hence God loves one soul in grace more than He loves all creatures with merely natural life, as, to illustrate, a father loves his children more than he loves his houses, and fields, his herds, flocks and droves. God, says St. Paul, guides the universe in favor of the elect.