“What does Paul mean when he says that whatsoever is not of faith is sin”?
“And he that doubteth is damned if he eat, because he eateth not of faith: for whatsoever is not of faith is sin” (Rom. 14:23).
We know that faith comes by hearing the Word of God (Rom. 10:17). The Word of God through Paul says that we can eat all things (1 Tim. 4:4), but he who is “weak in faith” (Rom. 14:1) doubts this and limits himself to eating “herbs” (Rom. 14:2). His faith has not yet matured to believe Paul when he says he can eat meat, so “he that doubteth… if he eat…he eateth not of faith.”
But if he wouldn’t eat it “of faith,” why would he eat it? Well, in this passage, he might eat meat trying to follow the example of his stronger brother. This is why Paul encourages strong brethren not to eat meat in front of a weaker brother (Rom. 14:15), which might make “the conscience of him which is weak be emboldened to eat” meat (cf. 1 Cor. 8:10). If he eats meat to try to walk in the footsteps of his stronger brother, rather than eating it because “of faith” in God’s Word, it will cause him to stumble (Rom. 14:13,21) by doing something that bothers his conscience.
Back to our question. How come “whatsoever is not of faith is sin”? It is because “to him that esteemeth any thing to be unclean, to him it is unclean” (Rom. 14:14). God actually adjusts the definition of what is unclean to agree with a weak brother’s conscience. Since his faith does not yet believe that he can eat meat, “he eateth not of faith,” and whatsoever is not of faith is sin to him.
Why would a weak brother “be damned if he eat”? Well, the word “damnation” doesn’t always refer to eternal damnation in Hell. If all damnation was eternal, the Lord was being redundant in speaking of “eternal damnation” (Mark 3:29). Likewise, if all damnation was to Hell, He would not have had to add the words “of Hell” when He spoke of “the damnation of Hell” (Matt. 23:33). The word “damnation” simply means condemnation or judgment of any kind. Those who resist the government “shall receive to themselves damnation” (Rom. 13:1,2), the judgment and condemnation of the government. So when a weak brother eats meat that he believes is unclean, it is sin for him, and he is condemned by his own conscience, since he judges what he has done to be sinful. source