Romans 14:23 - Matthew Poole's English Annotations on the Holy Bible

In this verse is another aphorism, respecting especially the weaker Christian. He that doubteth of the lawfullness of any meat, whether he may or may not eat it, is damned if he eat, i.e. His own conscience condemns him, or he makes himself liable to damnation, because he eateth not of faith. The word eateth is not in the original, but it is aptly inserted by our translators. What a man doth doubtingly, he doth sinfully: he showeth a wicked heart, that is not afraid of sin, but in great readiness to commit it. For whatsoever is not of faith is sin; this is a confirmation of the foregoing assertion. By faith here is meant knowledge, or full persuasion, as Romans 14:22: q.d. Whatever a man doth with a wavering mind, without being persuaded that it is pleasing to God, and warranted by his word, he sinneth in the doing of it. Though we may not nourish doubts and scruples, yet we must not act against them. An erring conscience binds us to act nothing contrary to it: he sins that doth any thing against it, though the fact or thing done should not be sinful. Nature itself teacheth as much: that is a known saying of Cicero: Quod dubitas, aequum sit an iniquum, ne feceris: If thou doubtest whether a thing be lawful, or not lawful, thou shalt not do it. See Hebrews 11:6.