Aquinas speaks to the relation between nature and grace concerning the sacraments. However, we should first understand the value of grace according to Aquinas and his predecessors.
In Question 2, Article 10, he references Augustine: “By the same grace every man is made a Christian, from the beginning of his faith, as this man from His beginning was made Christ.” Aquinas follows by underscoring the unity of nature and grace, “But this man became Christ by union with the Divine Nature. Therefore this union was by grace.” He continues in Question 7, Article 11 by stating that grace is taken in two ways: the will of God graciously given and as the free gift of God. Finally, he speaks to the unending effect of grace because of its unity with the Divine Nature.
Aquinas later addresses sacrament and grace in Question 38, Article 6. He again references Augustine in reminding us that “our sacraments are signs of present grace,” and he contrasts our present grace with the Old Law which were signs of a future grace.
Understanding the nature of grace and what a sacrament is a signifier of, we can comprehend the value of grace within a sacrament. Aquinas again speaks of Augustine in Question 60, Article 4, “The word is added to the element and this becomes a sacrament.” He states that “sacred things which are signified by the sacraments are the spiritual and intelligible good by means of which man is sanctified, it follows that the sacramental signs consist in sensible things.”
Aquinas continues in his expression of sacraments by underscoring their need within the life a believer. He responds to this need in Question 61, Article 1 (again, echoing Augustine) “sacraments are necessary for man’s salvation.” Aquinas moves further in the need for the sacraments within the economy of salvation by detailing three reasons within that response to the initial objection.
Finally, Aquinas addresses the effect of the sacraments: grace. He says in Question 62, Article 1 that it is the sacraments that both signify and cause grace. Furthermore, in that same Question in Article 2, Aquinas states that grace is in the sacrament and “understood as the instrument of some work to be done.”
Aquinas essentially states that a sacrament is given its power through grace and by grace for the purpose of bestowing grace upon and within the believer. And it is because of the humanity and divinity of Christ in both natures that make this movement of grace possible.