Significantly, it was to Paul, not Peter, that “the secret of the gospel” was first revealed. (See Eph. 3:1-3; 6:19). It was he who was first sent forth to proclaim the doctrine of salvation, and to reveal all that had been accomplished at Calvary.
The Old Testament Scriptures had predicted that the sins of others would be laid upon Christ, but they had not explained how Christ’s death would be the basis for the sinner’s justification.
Many a criminal has gone free because his crimes have been “pinned on” another, but this has by no means justified the criminal! Some sincere Christians seem to think that substitution is the very acme of Bible truth, when in fact it is but the beginning, for substitution in itself does not imply the sinner’s justification.
It is also true that salvation had been offered before Paul. Men were told what to do to be saved — though the terms varied from time to time — and were even instructed, upon Christ’s arrival, to believe in Him for salvation. At that time sacrifices, circumcision, water baptism, etc., were still required for the remission of sins — and any believer would approach God in His way. This is why these religious rites were observed throughout our Lord’s earthly ministry and even through Pentecost.
The Apostle Paul, however, was later raised up to make known “the secret of the gospel,” and to proclaim the glorious accomplishments of Christ at Calvary. All the rich blessings so thrillingly set forth in Paul’s epistles flow to us from Calvary. Ours is a heavenly position because He came to earth to die for our sins. Ours is “the hope of glory,” because He suffered our shame. Ours is the blessing of “peace with God” because He bore God’s wrath upon sin. Ours is relief from the load of sin because He bore that load. Every one of our “all spiritual blessings” (Eph. 1:3) comes to us from Calvary. Paul’s “secret of the gospel” centers in Calvary. Little wonder St. Paul calls his preaching “the preaching of the cross” (1 Cor. 1:18). source