On English presentation of St. Paul’s teaching is in this point very misleading. In the New Testament the word adoption is used only by the apostle Paul. The Greek word might be variously meant; the English can mean but one thing, and that is not what St. Paul means. “The spirit of adoption” Luther translates “The spirit of a child;” adoption he translates kindschaft, or childship. The word used by St Paul does not imply that God adopts children that are not his own, but rather that a second time he fathers his own; that a second time they are born—this time from above; that he will make himself tenfold, yea, infinitely their father: he will have them back into the very bosom whence they issued; he will have them one with himself. It was for the sake of this that, in his Son, he died for them.
Consider, in the apostle’s letter to the Galatians, 4:1-7, the last line of which reads “So that thou art no longer a bondservant, but a son; and if a son, then an heir through God.” It is as plain as St. Paul could make it that, by the word translated adoption, he means the raising of a father’s own child from the condition of subjection to others—a state which, he says, is no better than that of a slave—to the position and rights of a son. None but a child could become a son—the idea is a spiritual coming of age; only when the child is a man is he really and fully a son.