I mentioned Acts 10 about God leading Peter to preach The Gospel to Gentiles. To the Jews, like Peter, The Gospel going to the Gentiles also, certainly WAS about the 'mystery' Paul later spoke of, which the OT prophets first accounted of.
This verifies your doctrine is false...
Acts 11:4-18
4 But Peter rehearsed the matter from the beginning, and expounded it by order unto them, saying,
5 "I was in the city of Joppa praying: and in a trance I saw a vision, A certain vessel descend, as it had been a great sheet, let down from heaven by four corners; and it came even to me:
6 Upon the which when I had fastened mine eyes, I considered, and saw fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air.
7 And I heard a voice saying unto me, 'Arise, Peter; slay and eat.'
8 But I said, 'Not so, Lord: for nothing common or unclean hath at any time entered into my mouth.'
9 But the voice answered me again from heaven, 'What God hath cleansed, that call not thou common.'
10 And this was done three times: and all were drawn up again into heaven.
11 And, behold, immediately there were three men already come unto the house where I was, sent from Caesarea unto me.
12 And the spirit bade me go with them, nothing doubting. Moreover these six brethren accompanied me, and we entered into the man's house:
13 And he shewed us how he had seen an angel in his house, which stood and said unto him, 'Send men to Joppa, and call for Simon, whose surname is Peter;
14 Who shall tell thee words, whereby thou and all thy house shall be saved.'
15 And as I began to speak, the Holy Ghost fell on them, as on us at the beginning.
16 Then remembered I the word of the Lord, how that He said, 'John indeed baptized with water; but ye shall be baptized with the Holy Ghost.'
17 Forasmuch then as God gave them the like gift as he did unto us, who believed on the Lord Jesus Christ; what was I, that I could withstand God?
18 When they heard these things, they held their peace, and glorified God, saying, 'Then hath God also to the Gentiles granted repentance unto life.'
KJV
Acts 15:7-11
7 And when there had been much disputing, Peter rose up, and said unto them, "Men and brethren, ye know how that a good while ago God made choice among us, that the Gentiles by my mouth should hear the word of the gospel, and believe.
8 And God, Which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as He did unto us;
9 And put no difference between us and them, purifying their hearts by faith.
10 Now therefore why tempt ye God, to put a yoke upon the neck of the disciples, which neither our fathers nor we were able to bear?
11 But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they.
KJV
Acts 15:13-17
13 And after they had held their peace, James answered, saying, "Men and brethren, hearken unto me:
14 Simeon hath declared how God at the first did visit the Gentiles, to take out of them a people for His name.
15 And to this agree the words of the prophets; as it is written,
16 'After this I will return, and will build again the tabernacle of David, which is fallen down; and I will build again the ruins thereof, and I will set it up:
17 That the residue of men might seek after the Lord, and all the Gentiles, upon whom My name is called, saith the Lord, Who doeth all these things.'
KJV
You need to quite making up fibs and heed the Word of God as written.