Romans 11 - Is Israel the Jews or all believers or something else?

"A partial hardening has come upon Israel, until the fullness of the Gentiles has come in. And in this way all Israel will be saved, as it is written," - Romans 11:25b-26a

Today I'm more just curious, in Romans 11 do you interpret Israel as the Jews or as the body of Christ (all believers) or something else? I've been curious lately to find many people take different stances on this.

Thoughts? And reasons?