The way I see it the mystery is that we are saved by God's grace through faith alone and the resulting indwelling of the Holy Spirit, and that this same faith has been what brings salvation since creation.
Of this church I was made a minister according to the stewardship from God bestowed on me for your benefit, so that I might fully carry out the preaching of the word of God, that is, the mystery which has been hidden from the past ages and generations, but has now been manifested to His saints, to whom God willed to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory. We proclaim Him, admonishing every man and teaching every man with all wisdom, so that we may present every man complete in Christ. (Colossians 1:20-28)
“For this reason I, Paul, the prisoner of Christ Jesus for the sake of you Gentiles–if indeed you have heard of the stewardship of God’s grace which was given to me for you; that by revelation there was made known to me the mystery, as I wrote before in brief. By referring to this, when you read you can understand my insight into the mystery of Christ, which in other generations was not made known to the sons of men, as it has now been revealed to His holy apostles and prophets in the Spirit; to be specific, that the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel, of which I was made a minister, according to the gift of God’s grace which was given to me according to the working of His power. To me, the very least of all saints, this grace was given, to preach to the Gentiles the unfathomable riches of Christ, and to bring to light what is the administration of the mystery which for ages has been hidden in God who created all things;” (Ephesians 3:1-9) This was in accordance with the eternal purpose which He carried out in Christ Jesus our Lord, (Ephesians 3:11)
"So in Christ Jesus you are all children of God through faith, for all of you who were baptized into Christ have clothed yourselves with Christ.There is neither Jew nor Gentile, neither slave nor free, nor is there male and female, for you are all one in Christ Jesus. If you belong to Christ, then you are Abraham’s seed, and heirs according to the promise." (Galatians 3:26-29)
All believers are Abraham's offspring through His "seed" Jesus Christ, They are the heirs according to promise.
"The promises were spoken to Abraham and to his seed. Scripture does not say "and to seeds," meaning many people, but "and to your seed," meaning one person, who is Christ." (Galatians 3:16)
“Just as Abraham “believed God, and it was accounted to him for righteousness.” Therefore know that only those who are of the faith are sons of Abraham. And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, “In you all the nations shall be blessed.” So then those who are of faith are blessed with believing Abraham.” (Galatians 3:6-9)
"Now, brothers and sisters, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain. For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures," (1 Corinthians 15:1-4)