How can I show that Christians are spiritual Israel?

You can’t, because you’re not.

The Christian bible does not get to REdefine the Jewish covenant with G-d. The Jewish covenant with G-d is precisely that: Jewish.

Christians have their OWN relationship with G-d – be content in that and stop trying to invalidate ours. What you are doing is called ‘replacement theology’ and it has been the cause of much anti Jewish sentiment through the ages.

Jews and Christians each have their own, special connection to G-d.