The Gospel message was not the mystery. Many saints were saved by hearing and believing the Gospel promises of a Savior.
Paul's gospel was a mystery hidden in the scriptures until given by the revelation of Jesus Christ!
Romans 16:25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith:27 To God only wise, be glory through Jesus Christ for ever. Amen.
Galatians 1:11 But I certify you, brethren, that the gospel which was preached of me is not after man. 12 For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ.
It was a mystery: for had the princes of this world known it, they would not have crucified the Lord of glory.
1 Corinthians 2:6 Howbeit we speak wisdom among them that are perfect: yet not the wisdom of this world, nor of the princes of this world, that come to nought: 7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: 8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
But, the fact that Paul was given a dispensation of the grace of God with that gospel to us-ward (Gentiles such as we Ephesians 2:11-12), was a mystery, not hidden in the scriptures ("according to the scriptures") as the gospel of Christ, but hid in God! It was unsearchable! It is the manifold wisdom of God!
Ephesians 3:1 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, 2 If ye have heard of the dispensation of the grace of God which is given me to you-ward: 3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words, 4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; 6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: 7 Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power. 8 Unto me, who am less than the least of all saints, is this grace given, that I should preach among the Gentiles the unsearchable riches of Christ; 9 And to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God, who created all things by Jesus Christ: 10 To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God, 11 According to the eternal purpose which he purposed in Christ Jesus our Lord:12 In whom we have boldness and access with confidence by the faith of him.
Trust the Lord believing 1 Corinthians 15:1-4 and be saved!