John Gill Commentary Romans 14:2

"One man hath faith to eat all things: but he that is weak eateth herbs." — Romans 14:2 (ASV)

For one believes that he may eat all things
He is fully persuaded in his mind, that there is nothing in itself common, or unclean; that the difference between clean and unclean meats, commanded to be observed by the law of Moses, is taken away; and that he may now lawfully eat any sort of food; every creature of God being good, and none to be refused, because of the ceremonial law which is abrogated, provided it is received with thanksgiving, and used to the glory of God:

another who is weak eats herbs ;
meaning not one that is sickly and unhealthful, and of a weak constitution, and therefore eats herbs for health's sake; but one that is weak in the faith, and who thinks that the laws concerning the observance of meats and drinks are still in force; and therefore, rather than break any of them, and that he may be sure he does not, will eat nothing but herbs, which are not any of them forbidden by the law: and this he did, either as choosing rather to live altogether on herbs, than to eat anything which the law forbids; or being of opinion with the Essenes among the Jews, and the Pythagoreans among the Gentiles, who thought they were to abstain from eating of all sorts of animals.