Charles Ellicott Commentary Romans 14:23

"But he that doubteth is condemned if he eat, because [he eateth] not of faith; and whatsoever is not of faith is sin." — Romans 14:23 (ASV)

And he who doubts.—The one thing that justifies a person in neglecting such fine and meticulous distinctions is a faith so strong that it can afford to treat them lightly. Where faith is not strong enough for this, and where the conscience deliberately approves one course but the other course is chosen, this alone marks the act as wrong. He who hesitates about what he ought to do is condemned, or does wrong, if he eats (in opposition to his conscience), because he does not have the one faculty that can overrule the decisions of conscience and give them a different direction.

Whatsoever is not of faith is sin.—This is intended as a general principle, but only as a general principle covering this particular kind of case. Where the conscience is in doubt, faith alone can make it right to choose the side against which conscience inclines. Nothing is said about those cases in which conscience is either not appealed to at all, or approves what is done.

Therefore, Saint Augustine was wrong in arguing from this passage that even good actions, when done by unbelievers, were of the nature of sin.