There was nothing mysterious about God saving the rest of humanity if they would believe.
There most certainly was for those who were heirs according to the promise (Galatians 3:29 KJV like the Romans, Corinthians, Galatians, Thessalonians). It came in the form of Paul's "gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, but now is made manifest,..."
Romans 16:25-27 KJV
And don't forget that through your ignorance, you are leaving us entirely out as we were Ephesians 2:11-12 KJV in time past, but now Ephesians 2:13 KJV! And it too was most certainly a mystery, but this one not hidden in the scriptures, but hid in God, unsearchable!
Ephesians 3:1 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
Ephesians 3:2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
Ephesians 3:3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
Ephesians 3:4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
Ephesians 3:5 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;
Ephesians 3:6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Ephesians 3:7 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.
Ephesians 3:8 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;
Ephesians 3:9 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:
Ephesians 3:10 To the intent that now unto the principalities and powers in heavenly places might be known by the church the manifold wisdom of God,
Ephesians 3:11 According to the eternal purpose which he purposed in Christ Jesus our Lord:
Ephesians 3:12 In whom we have boldness and access with confidence by the faith of him.