Lit. 'to (eis) one flesh.' In English 'shall' or 'shall become' is the nearest in sense. It corresponds to the Hebrew of Gen. 2.24. Eis is left out when he says 'one Spirit' in ver. 17. We are really 'one Spirit,' not two, with the Lord. But we cannot say, 'to' or 'for one flesh.' The two become so (i.e. 'one flesh') practically by their union; they are created individually. The union induces unity in the flesh; 'shall be,' or 'shall become' partly, though imperfectly, implies this: see Matt. 19.5. It is not 'shall be one Spirit:' but 'he is.' The Spirit which is in the Lord himself dwells in us, and is the living power of the new life.