Paul carefully demonstrates, in opposition to the Judaizers, that the method by which Abraham obtained salvation is the type, the model of the method in accordance with which the believers in the New Testament must believe, whether they be Jews or pagans. Abraham is the father of believers (Rom. iv. 9-25, Gal. iii. 7-9, 17 f., 26-29). The law has neither annulled nor altered the covenant of grace (cf. Heb. vi. 13-18). Such being the nature of the covenant with Abraham, it is clear that, in the degree which is has reference to what is of essential nature, it remains always in force. We still live today under this covenant of grace; the Christian Church is founded upon it. Even at this present moment man is justified by faith just as Abraham was. Christians are called the children and heirs of Abraham, because faith in the promise of redemption assures their redemption exactly as faith in the same promise assured Abraham’s redemption. “If you are Christ’s,” says Paul, “then you are Abraham’s posterity and heirs according to promise” (Gal. iii.29).
…For this reason this reason it is necessary to understand the Old Testament a being one in essence and substance with the New. Even though God communicates His revelation to us in the course of the unfolding of history and in a progressive manner, and thought consequently mankind progresses in the knowledge, possession, and enjoyment of the revelation, yet God is always identical with Himself, and remains so. (Pierre Marcel in Infant Baptism. All punctuation, etc. is his.)