Romans 11:1 — I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin.
Paul was a Jew, so his salvation proves God has not cast off Israel
Seed of Abraham — the man God made promises to
Tribe of Benjamin — the tribe, with Judah, that was saved from destruction (Deuteronomy 21) and remained faithful after the captivity (Ezra 4:1, 10:9).
To the opening question, "Did God cast off His people?" there are three replies, which cover the ground of the whole chapter. Israel is not utterly given up, for (1) Paul himself is an Israelite, and therefore a spiritual remnant exists (vs.1-10); (2) one of the reasons why God is calling Gentiles into His favor is in order that He may show mercy to Israel; so He could not have rejected His people (vs. 11-24); (3) God will fulfil His promise to the nation on the coming of their Deliverer (vs. 25 to the end). — W.E. Vine, The Epistle to the Romans, page 161-162.
We object to this whole system of interpretation [that Israel has lost her chance forever]. It is as illegitimate as if a man should promise to give his son a gold watch on his birthday but, grieved with his son before the birthday arrives, he "keeps his promise" by giving a grandfather clock to another person altogether! — C.R. Stam, The Epistle of Paul to the Romans — page 260.