Which promise to Abraham is guaranteed no matter what happens to Israel?
The guaranteed promise to Abraham is that through him all nations of the earth would receive a blessing, regardless of what happens to Israel. This was fulfilled when Christ came and finished the work of the cross, becoming a blessing out of Abraham's line for the whole human race.
Les explains that while empires and nations rise and fall, and while Israel's circumstances change, one Abrahamic promise remains secure: through the man Abraham, all nations of the earth would receive a blessing. He teaches that this came about through Israel's Messiah. When Christ came and finished the work of the cross, He became a blessing out of Abraham's line, not only for Israel but for the whole human race. Les also links this to Romans 11, where natural Israel was broken off and the Gentiles were grafted in.