The sacraments work in two ways. One way is by actually being administered. The other way is by being desired. The reason for this is that the sacraments are instruments of God's justifying mercy, and God sees the human heart....Hence, even though natural things have no effect without being presently employed, the sacraments have an effect just by being desired. But they have their fullest effect when the sacraments are actually administered.
[Thomas Aquinas, Quodlibet IV, Question 7, Article 1, from Thomas Aquinas, Quodlibetal Questions, Nevitt & Davies, trs., Oxford University Press (New York: 2020) p. 352. While the immediate topics are baptism and penance, Aquinas explicitly states later that this applies to the other major sacraments.]
