The Gospel of Jesus Christ, that is, the gracious message of the forgiveness of sins through faith in the promised Savior, was the divine means of grace also during the whole time of the Old Testament, Acts 15, 11. This is the clear testimony of Scripture, which tells us: "To Him give all the prophets witness that through His name whosoever believeth in Him shall receive remission of sins," Acts 10, 43. In the promised Christ, Abraham believed, John 8, 56, and of Him Moses wrote, John 5, 46. St. Paul expressly assures us that all New Testament believers are "Abraham's children," Gal. 3, 7, and "Abraham's seed," Gal. 3, 29, because they believe as Abraham did.
In particular, the New Testament Scriptures declare that the Christian doctrine of justification by grace, through faith, without the deeds of the Law, is "witnessed by the Law and the prophets," Rom. 3, 21. The entire fourth chapter of the Epistle to the Romans is designed to prove that the doctrine of justification by faith is a doctrine of the Old Testament. Even after the Mosaic covenant was established, the Gospel of Christ as a means of grace was still in force, Gal. 3, 17.
It is true, when the promised Messiah appeared in the fulness of time, the Jews did not believe in Him; but this was not due to lack of adequate testimony concerning Him, but to their contempt for the clear witness of Moses, John 5, 45-47. So also the weak faith of the disciples in the crucified and risen Savior was owing to their disregard of the clear prophecies of the Old Testament, Luke 24, 25. For the same reason also modern rationalistic theology denies the incontestable fact that ever since the Fall the Gospel of Christ has been the true means of grace by which sinners have become children of God through faith in the divine promise. It repudiates the Messianic character of clear Messianic prophecies, Gen. 3, 15; 4, 1; etc., just because it refuses to believe the witness of the prophets and, what is worse, that of Christ Himself and of His holy apostles, Luke 24, 25. In short, rationalism cannot find Christ and His vicarious atonement in the Old Testament because it does not believe the Gospel so clearly set forth in the New Testament.
As the Gospel of Christ, so also the Circumcision and the Passover were means of grace, offering and conveying forgiveness of sins. To the act of circumcision was attached the divine promise of grace: "I will be their God," Gen. 17, 8, that is, their gracious God, who out of pure love freely forgives sin. This is evident from the fact that in the New Testament St. Paul calls the sign of circumcision "a seal of the righteousness of faith," Rom. 4, 11. Of the Passover, Scripture distinctly says that it mediated grace to the Israelites; for they were spared in the plague, not as Jews, but because the passover lamb was killed and its blood was sprinkled on the lintel and the two side-posts, Ex. 12, 21-27. For this reason God commanded: "And ye shall observe this thing for an ordinance to thee and to thy sons forever," Ex. 12, 24. Hence both to Circumcision and to the Passover was attached the divine promise of grace, and we therefore rightly speak of them as the Sacraments of the Old Testament.
Luther writes: "It is a mistake to believe that the Sacraments of the New Testament differed from the Sacraments of the Old Testament according to their force and meaning namely, as divinely appointed means of grace. . . . Both our and the fathers' signs, or Sacraments, have attached to them a word of promise, which demands faith and cannot be fulfilled by any other work. Therefore they are signs, or Sacraments, of justification." St. L., XIX, 62 ff.{:target="_blank"} Thus also in the Old Testament the Gospel and the Sacraments (Circumcision and the Passover) offered and conveyed to the believers divine grace and forgiveness. In other words, their function was precisely the same as is that of the means of grace instituted by Christ in the New Testament.