This is said only in passing, but it's still worth mentioning. While discussing "Whether Baptism is the mere washing?", Aquinas says in response to an objection:
That which is both sacrament and reality--i.e. the character--and that which is reality only--i.e. the inward justification--remain: the character remains and is indelible, as stated above (Question 63, Article 5); the justification remains, but can be lost. [ST III, Q66, A1, ad 1; emphasis added]
This is an inevitable consequence of the fact that man has a free will: he may choose to reject Christ after having been baptized. It is a dreadful and terrible thing.