"Sanctifying grace, to proceed, is a permanent quality of the soul. It is the living water, springing up into eternal life. [1107] It is "the seed of God," [1108] which tradition calls "the seed of glory." [1109] St. Thomas [1110] formulates a precise doctrine, which found ever wider acceptance and final approval in the Council of Trent. [1111] 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. [1112] 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. [1113] 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."
Reality - A Thomistic Synthesis: by Pere Reginald Garrigou-Lagrange, O.P. - Complete book online
So to put this together.. though sanctifying grace is created, it disposes us to participate truly in God's nature. But we remain creatures. Sanctifying grace is accidental. But our participation is in God's actual nature, not just in a creature. The created grace is to dispose us towards this.
the light is still the uncreated divine nature. I hope that makes sense. I need to look up how he described it - I added the details in the analogy myself, but the example is St John's.
