Peter told them that the gospel that saved them was to repent and be baptized for the remission of sins in Act 2:38.
They were repenting of their UNBELIEF in their prophesied Messiah, and being WATER baptized as a REQUIREMENT, or demonstration of their FAITH in THAT FACT. This is how they were saved , or justified under the Kingdom Program. Of course, in GODS foreknowledge, HE SAW that it was the blood of Christ not yet shed that would actually save them, but that blood , ie: the DEATH< BURIAL, and RESURRECTION is NOT the message given to them that they were required to believe. They knew of the EVENT, but of the PURPOSE of the Cross and the preaching of the Cross as what saves, that was NOT preached UNTIL the "preaching of Jesus Christ ACCORDING TO THE MYSTERY which was kept SECRET since the world began..." Romans 16:25.
Please keep in mind that under the KINGDOM GOSPEL the Jews were NOT separated from God, but were betrothed unto Him. She was His wife. The covenant was a marriage contract between the Nation Israel and God Himself. It is NOT UNTIL God set them ASIDE for UNBELIEF, and took away their "favored status" that Israel became alienated as the rest of the nations. That is why God says "He concluded them ALL (gentile AND JEW) in UNBELIEF, so He could have mercy upon all." There was no reconciliation for the world UNTIL Israels favored status was SET ASIDE, and God ushered in the MYSTERY, which in ages and generations past was HID.
I hope this helps.