No, I don't have to do any such thing.As I say you have to first show us Israelites who had the free gifts of justification and eternal life, but who then did not believe anymore to prove your thesis that he's talking about those two specific gifts in regard to the Israelites in Romans 11:28-29. Can you do that? I don't think so.
Here are the facts from Romans:
1. Eternal life is a gift - 6:23
2. God's gifts are irrevocable - 11:29
If eternal life is revocable (your position), please prove it from Romans, where Paul wrote both 6:23 and 11:29.
If Paul excluded eternal life from Rom 11:29, the burden is on you to prove it from Romans.
The OP has pointed out 2 FACTS. Which fact do you disagree with or reject?