"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)
"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" (Zech. 2:11)These passages show that to join with the L-rd means to join the covenantal People of Israel (see Cohen below). But what is meant by "joining" with the L-rd? Is this accomplished through faith or by circumcision?
In Acts 15, James the Apostle connects these passages--via gezerah shava--to Amos 9:11-12
"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 as it appears in LXX)This proves, if nothing else, that James believed that "joining" the Lord (and therefore joining Israel) can be accomplished WITHOUT circumcision.
"Isaiah 56 refers to the foreigners who have 'attached themselves to the Lord...'....Zechariah [2:11] predicts that 'in that day many nations will attach themselves to the Lord and become his people.'.....these passages show that in the Persian period...comes the beginning of the idea that gentiles could somehow attach themselves to the people of Israel by attaching themselves to Israel's God," (pg. 122 of Cohen's 'The Beginnings of Jewishness).