One of the most important distinctions found in the New Testament is that between things which were foretold in the OT and things which were kept secret. With that in mind let us look at the following verse:
"Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God, (Which he had promised afore by his prophets in the holy scriptures) Concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh" (Ro.1:1-3).
Sir Robert Anderson says, "Strange it seems that Expositors should have failed to notice the clearly marked difference between the gospel of the opening verses of Romans and that of the Apostle's postscript at its close. We read the epistle amiss if we fail to notice what an important place its teaching accords to the Hebrew Christians, who doubtless were the majority in the local church. For in early days it was to Jews only that the gospel was preached; and the word which had won them was the gospel of God, which He promised afore by His prophets in the Holy Scriptures concerning His Son 'who was born of the seed of David.' This was the hope of every true Israelite. In keeping with it were the Apostle's words to the chief of the Jews in Rome: 'For the hope of Israel I am bound with this chain.' And his answer to the charge on which he was imprisoned was that his preaching to the Jews was based entirely on their own Scriptures (Acts xxvi. 22)" (Anderson, Misunderstood Texts of the New Testament [Grand Rapids: Kregel Publications, 1991], 82).
Paul's was imprisoned for being a "mover of sedition among all Jews throughtout the world" (Ro.24:5) so anything which he said in defense of his ministry was based on what he taught among the Jews and nothing more. In his defense before king Agrippa he said:
"Having therefore obtained help of God, I continue unto this day, witnessing both to small and great, saying none other things than those which the prophets and Moses did say should come: That Christ should suffer, and that he should be the first that should rise from the dead, and should shew light unto the people, and to the Gentiles" (Acts 26:22-23).
Indeed, the gospel which Paul preached unto the Jews was based on nothing except the things previously revealed in the OT Scriptures. On the other hand, the gospel which he preached to the Gentiles was based on a "mystery" truth--something which had been kept secret and therefore was not revealed in the OT:
"Now to him who is able to establish you in accordance with my gospel, the message I proclaim about Jesus Christ, in keeping with the revelation of the mystery hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God, so that all the Gentiles might come to the obedience that comes from faith" (Ro.16:25-26; NIV).
Anderson also says that "the gospel which he preached to Gentiles he had received by special revelation to himself ; and to communicate that gospel to his brother Apostles was the purpose of his third visit to Jerusalem (Gal.ii:2)" (Ibid.).
Acts 2 dispensationalist Lewis Sperry Chafer also understood that the Twelve Apostles did not understand the meaning of the "new age" and that revelation did not come unto them until much later at the first Church Council when they met with Paul:
"It was not until Peter by divine compulsion had first preached the Gospel to Gentiles in Cornelius' house, and Paul and Barnabas had returned to Jerusalem reporting the same out flowing salvation to Gentiles as had been given to Jews that they were able to grasp the meaning of the new age. This new light came in connection with the deliberations of the first church council, called by the mother church at Jerusalem, and recorded in Acts 15:13-18" [emphasis added] (Chafer, The Church Which is His Body).
The gospel which Paul preached to the Gentiles was kept secret and was not revealed to the Twelve is also spoken of here by Paul:
"And I, brethren, when I came to you, came not with excellency of speech or of wisdom, declaring unto you the testimony of God. For I determined not to know any thing among you, save Jesus Christ, and him crucified...we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory" (1 Cor.2:1-2, 7-8).
Anderson continues, saying that "there can be no doubt that the 'mystery' of our verse is what the Apostle calls elsewhere 'the mystery of the gospel' - the reign of grace, which is the great basal truth of the distinctly Christian revelation - a truth which was not, and obviously could not be, declared until the covenant people were set aside. For grace is as incompatible with covenant, or special favour of any kind, as it is with works" (Ibid.).
It is the "gospel of grace" which Paul said went into all the world and that those who received his epistle "knew the grace of God in truth":
"For the hope which is laid up for you in heaven, whereof ye heard before in the word of the truth of the gospel; Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth" (Col.1:5-5).