God adopts men as children, in as much as through his infinite bounty, he admits them to the participation of his inheritance, to which naturally they would have no right. But in what does the inheritance of God consist?
We creatures need some addition to have a treasure. But God does need anything outside of himself, therefore the inheritance cannot be some mere creature – for everything beyond God is a creation of God, something less than God:
By an inheritance of some one is meant those goods which form his riches. Now that which forms the riches of God, is the fruition of God himself; for God needs not other goods outside of himself, but is rich in himself and of himself, and is completely happy by the enjoyment of himself.
What then would be the inheritance of God:
Therefore the inheritance of God is the fruition of God, that is to say, the happiness proper of God.
How then may may creatures partake of the happiness of God, when we are so unfit to receive it
And when he adopts men as children, he admits them to share in this happiness, which is natural to himself, but supernatural to them. And since man of himself is unable to attain such beatitude which transcends the power of his nature, God himself renders him capable through the gift of grace. Hence we see how much superior the divine adoption is to the human one. Man does not make the adopted fit but supposes him such, and therefore adopts him; whilst God, on the contrary, supposes man to be not fit, and, by adopting him, renders him fit for the attainment of the heavenly inheritance.
From Chapter III, Jesus Christ the Word Incarnate.