But he that is joined to the Lord - The true Christian, united by faith to the Lord Jesus; see John 15:1 ff.
Is one spirit - That is, in a sense similar to that in which a man and his wife are one body. It is not to be taken literally; but the sense is, that there is a close and intimate union; they are united in feeling, spirit, intention, disposition. The argument is beautiful. It is, “As the union of souls is more important than that of bodies; as that union is more lasting, dear, and enduring than any union of body with body can be, and as our union with him is with a Spirit pure and holy, it is improper that we should sever that tie, and break that sacred bond, by being joined to a harlot. The union with Christ is more intimate, entire, and pure than that can be between a man and woman; and that union should be regarded as sacred and inviolable.” O, if all Christians felt and regarded this as they should, how would they shrink from the connections which they often form on earth! Compare Ephesians 4:4.