Albert Barnes on Romans 14:1

Him that is weak. The design here is to induce Christians to receive to their fellowship those who had scruples about the propriety of certain things, or that might have peculiar prejudices and feelings as the result of education or former habits of belief. The apostle, therefore, begins by admitting that such an one may be weak, i.e., not fully established, or not with so clear and enlarged views about Christian liberty as others might have.

In the faith. In believing. This does not refer to saving faith in Christ, for he might have that; but to belief in regard to the things which the apostle specifies, or which would come into controversy. Young converts have often a peculiar delicacy or sensitiveness about the lawfulness of many things in relation to which older Christians may be more fully established. To produce peace, there must be kindness, tenderness, and faithful teaching; not denunciation, or harshness, on one side or the other.

Receive ye. Admit to your society or fellowship; receive him kindly, not meet with a cold and harsh repulse. Comp. Rom 15:7.

Not to doubtful disputations. The plain meaning of this is, "Do not admit him to your society for the purpose of debating the matter in an angry and harsh manner; of repelling him by denunciation; and thus, by the natural reaction of such a course, confirming him in his doubts." Or, "do not deal with him in such a manner as shall have a tendency to increase his scruples about meats, days, etc." (Stuart.) The leading idea here--which all Christians should remember--is, that a harsh and angry denunciation of a man in relation to things not morally wrong, but where he may have honest scruples, will only tend to confirm him more and more in his doubts. To denounce and abuse him will be to confirm him. To receive him affectionately, to admit him to fellowship with us, to talk freely and kindly with him, to do him good, will have a far greater tendency to overcome his scruples. In questions which now occur about modes of dress, about measures and means of promoting revivals, and about rites and ceremonies, this is by far the wisest course, if we wish to overcome the scruples of a brother, and to induce him to think as we do.--Greek, "Unto doubts or fluctuations of opinions or reasonings." Various senses have been given to the words, but the above probably expresses the true meaning.