“And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin.”
King James Version (KJV)
14:23 Because it is not of faith - He does not believe it lawful and, in all these cases, whatsoever is not of faith is sin - Whatever a man does without a full persuasion of its lawfulness, it is sin to him.
Ro 14:23 He that doubteth is condemned if he eateth. He is contrasted with him "who has faith" (Ro 14:22). He has not faith, or does not believe that it is right to eat these meats. Hence he is "condemned" ("damned") by his own conscience. Whatsoever [is] not from faith is sin. The context shows that Paul means that whenever actions are done by a Christian which he does not believe are right, he sins in doing them. If he is doubtful whether they are right, he must not do them.