How could it all be one when it was before a mystery (Romans 16:25-27 KJV, 1 Corinthians 2:6-8 KJV)? The answer is that it couldn't have been the same! Drop your religion and believe the Bible!
Yes, yes, Paul speaks often of the gospel as a mystery ‘hidden for long ages past but now revealed’, which is revealed to him and made known through the proclamation of the gospel (e.g., 1 Cor 2:1, 7; Eph 1:9; 3:3-4, 9; 6:19; Col 1:26; 4:3).
But now in the Romans 16 doxology Paul adds that it is revealed and made known through the prophetic writings (the Old Testament) by the command of the eternal God, contrary to anti-covenantalists who regard the church age as being a mystery revealed to Paul and
not as something predicted by the Old Testament prophets. That these prophetic writings are from the Old Testament is clear from Romans 1:1-2, wherein Paul describes the gospel as something God promised beforehand through his prophets in the Holy Scriptures. As the gospel is preached God opens people’s eyes,
especially Jewish eyes, to see that the OT Scriptures testify to Christ (2 Cor 3:14-18; Luke 24:25-27, 44-47).
So that no one might raise a dispute on the subject, and charge the gospel with being a new thing, as do the anti-covenantalists, and thus defame it, Paul refers to the prophetic Scriptures, in which we now see,
that what is fulfilled had been foretold; for all the Prophets have rendered to the gospel a clear testimony, that it can in no other way be so fully confirmed. God thus duly prepared the minds of his people, lest the novelty of what they were not accustomed to should astonish them too much.
For those anti-covenantalists that would then raise up objection that Paul is being inconsistent by saying things have been kept secret yet have been foretold in the Old Testament, Peter unties the knot by noting that the Prophets, when they diligently inquired of the salvation made known to us, ministered, not to themselves, but to us (1 Peter 1:12). God then was at that time silent, though He spoke, for God held in suspense the revelation of those things concerning which He designed that His servants should prophesy.
Paul's words also were in regard to differences between the Old and the New Testament. Though the Prophets formerly taught all those things which have been explained by Christ and his Apostles, they taught them with much obscurity, that in comparison with the clear brightness of New Testament's gospel light, it should be no wonder that those things are said to have been hidden which are now made manifest.
To believers, Paul adds in the Romans 16 doxology that Christ is the wisdom of God (1 Cor 1:24, 30), for in him are hidden all the treasures of wisdom and knowledge (Col 2:3). For Paul the wisdom of God determines the way people will come to know him—not through human wisdom but through the preaching of the cross of Christ (1 Cor 1:21-24; 2: 67). Also, it is God’s intention that through the church, the manifold wisdom of God should be made known to the rulers and authorities in the heavenly realms (Eph 3: 10), that is, the manifold wisdom of God in the mystery of the gospel now revealed.
From the mystery spoke of it is evident that it means there is not a distinction between the Old and the New Testament as far as the way of salvation is concerned (one Gospel!), but rather the essence of the mystery was that one day the Gentiles would not only be entering God’s kingdom in large numbers but would be fellow-sharers, participants on equal terms, with the elect from among the Jews. Christ in you, the hope of glory (Col. 1:27) would be the solid basis for present salvation and future eschatological glory for
everyone, regardless of race, who would, by God’s sovereign grace, place his trust in Our Lord.
AMR