St. Thomas on Justification - Justification can be lost

More can be said about this subject than I will say in this post; I have probably faltered in this sometimes, but my purpose in this series isn't to provide my opinions about the subject matter, nor to expand on other things that might be said about it, but rather to stick to what St. Thomas has said as a preeminent representative of the Catholic Faith.

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.