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Re: "Amos 9:11-12 is unique in its clarity that they remain Gentiles and yet are called by God’s name."
A Gentile always remains a Gentile, even a convert. But such a Gentile is also part of the covenant--and that's all that matters. Thus, Caleb was a Gentile even though he was an Israelite. Same with Ruth (note that she's even called "Ruth the Moabite" despite belonging to the People of Israel).
In other words, in a contest between ethnicity (gentileness) and covenant, covenant always wins.
But don't take my word for it. Isaiah 63:19 says that uncovenanted Gentiles are not called by His Name:
"We are yours from of old; but you have not ruled over them, they have not been called by your name."
So we see that this idea of being called by someone's name refers to covenant:
"In that day seven women will take hold of one man and say, "We will eat our own food and provide our own clothes; only let us be called by your name. Take away our disgrace!"" (Isaiah 4:1)
Remember that Abraham was accepted in covenant with G-d even prior to circumcision. The circumcision is a sign and seal of the pre-existing covenant.
"And he received the sign of circumcision, a seal of the righteousness that he had by faith while he was still uncircumcised. So then, he is the father of all who believe but have not been circumcised, in order that righteousness might be credited to them," (Romans 4:11)
The covenant occurs apart from circumcision and thus salvation is possible apart from circumcision. And recall from Acts 15:1 that this was the issue: can Gentiles be in covenant apart from circumcision so that they may receive salvation even in an uncircumcised state? And the answer, of course, was "yes." Peter's argument before the council explained that the uncircumcised Gentiles had been called by His name and were, THEREFORE, saved by grace. Note that James says that Peter made this argument about the gentiles being called by the Name of G-d.