The question at hand in the Summa is whether every act of obedience to God must be done out of charity. St. Thomas responds:
Whoever breaks a commandment sins mortally. If therefore the mode of charity falls under the precept, it follows that whoever acts otherwise than from charity sins mortally. But whoever has not charity, acts otherwise than from charity. Therefore it follows that whoever has not charity, sins mortally in whatever he does, however good this may be in itself: which is absurd (ST I-II Q100 A10; emphasis added).I've talked about this sort of thing before. It's crazy to say that a non-Christian Boy Scout has sinned by helping a little old lady across the street. The fact that he does an objectively good thing doesn't mean that he can work his way to heaven, though.
Wherefore he that honors his father, yet has not charity, does not break this precept: although he does break the precept concerning the act of charity, for which reason he deserves to be punished (ST, ibid).We are saved by grace. But we don't have to treat the unbeliever as always doing evil, no matter what he does. It's just not so. The fact that we cannot save ourselves doesn't oblige us to think any such thing.