Him that is weak in the faith receive ye, but not to doubtful disputations. Him that is weak in the faith - By this the apostle most evidently means the converted Jew, who must indeed be weak in the faith, if he considered this distinction of meats and days essential to his salvation. See on Romans 14:21 (note).
Receive ye - Associate with him; receive him into your religious fellowship; but when there, let all religious altercations be avoided.
Not to doubtful disputations - Μη εις διακρισεις δια λογισμων. These words have been variously translated and understood. Dr. Whitby thinks the sense of them to be this; Not discriminating them by their inward thoughts. Do not reject any from your Christian communion because of their particular sentiments on things which are in themselves indifferent. Do not curiously inquire into their religious scruples, nor condemn them on that account. Entertain a brother of this kind rather with what may profit his soul, than with curious disquisitions on speculative points of doctrine. A good lesson for modern Christians in general.