So when did these Gentiles, who kept the Passover to the Lord, learn the (supposed hidden and mysterious) gospel of grace through Messiah? Or did they ever? Or will they ever?
There was a mystery given to Paul that YOU deny.
Rom 16:25-26 KJV 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:
You simply will not listen to the Lord Jesus Christ as He speaks through Paul.
Eph 3:1-12 KJV 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.
It is you that rejects the truth. Paul, over and over again, explains the uniqueness of the message given to him by the Lord Jesus Christ. Paul was given a REVELATION what was HID IN GOD.
You guys keep saying that it was ALWAYS KNOWN, and yet Paul said that it was
KEPT SECRET SINCE THE WORLD BEGAN.
I'm going to go ahead and stick with GOD'S MESSENGER, PAUL, on this one. I think that he knows a bit more than you.