"This mystery is that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus," (Eph 3:6)
"Therefore, remember that formerly you who are Gentiles by birth and called “uncircumcised” by those who call themselves “the circumcision” (which is done in the body by human hands)— remember that at that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world. But now in Christ Jesus you who once were far away have been brought near by the blood of Christ," (Eph 2:11-13)
Until the arrival of Paul the Apostle, most Messianic Jews thought of Yeshua's parting words "Go and make disciples of all nations" as some sort of miscommunication.
Why did they (and especially Peter) have such a hard time with this mystery of Gentile inclusion in Israel?
Because it seemed to contradict Torah! Just look at the following examples:
“You are a people (LXX, laos) holy to the LORD your God. Out of all the people (LXX, ethne) on the face of the earth, the LORD has chosen you to be His treasured possession.”(Deut. 14:2).
“We are yours from of old; but you have not ruled over them, they have not been called by your name.” (Isaiah 63:19)
"When they finished, James spoke up. “Brothers,” he said, “listen to me. Simon has described to us how God first intervened to choose a people for his name from the Gentiles. The words of the prophets are in agreement with this..." (Acts 15:13-15)
"11 In that day I will raise up the tabernacle of David that is fallen, and will rebuild the ruins of it, and will set up the parts thereof that have been broken down, and will build it up as in the ancient days: 12 that the remnant of men, and all the Gentiles upon whom my name is called, may earnestly seek me, saith the Lord who does all these things," (Amos 9:11-12 LXX)
"And many nations shall be joined to the Lord in that day, and shall be my people: and I will dwell in the midst of thee, and thou shalt know that the Lord of hosts hath sent me unto thee," (Zechariah 2:11)
"Neither let the son of the stranger, that hath joined himself to the LORD, speak, saying, The LORD hath utterly separated me from his people..." (Isaiah 56:3)
"Also the sons of the stranger, that join themselves to the LORD, to serve him, and to love the name of the LORD, to be his servants, every one that keepeth the sabbath from polluting it, and taketh hold of my covenant; Even them will I bring to my holy mountain, and make them joyful in my house of prayer: their burnt offerings and their sacrifices [shall be] accepted upon mine altar; for mine house shall be called an house of prayer for all people," (Isaiah 56:6-7)
"And it shall come to pass in the last days, [that] the mountain of the LORD’S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. Andmany people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem," (Isaiah 2:2-3)
"But in the last days it shall come to pass, [that] the mountain of the house of the LORD shall be established in the top of the mountains, and it shall be exalted above the hills; and people shall flow unto it. And many nations shall come, and say, Come, and let us go up to the mountain of the LORD, and to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for the law shall go forth of Zion, and the word of the LORD from Jerusalem," (Micah 4:1-2)
"31 Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel, and with the house of Juda: 32 not according to the covenant which I made with their fathers in the day when I took hold of their hand to bring them out of the land of Egypt; for they abode not in my covenant, and I disregarded them, saith the Lord. 33 For this is my covenant which I will make with the house of Israel; after those days, saith the Lord, I will surely put my laws into their mind, and write them on their hearts; and I will be to them a God, and they shall be to me a people," (Jeremiah 31:31-33 LXX)
"Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you. A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh. And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do [them. ]" (Ezekiel 36:25-27)
Here is the prophecy to which James refers in Acts 15:18-19
"If they will declare, let them draw nigh, that they may know together, who has caused these things to be heard from the beginning: then was it told you. I am God, and there is not another beside me; a just God and a Saviour; there is none but me. 22 Turn ye to me, and ye shall be saved, ye that come from the end of the earth: I am God, and there is none other," (Isaiah 45:21-22 LXX)