Would you extend the meaning of this passage -- "2 Hereby know ye the Spirit of God: Every spirit that confesseth that Jesus Christ is come in the flesh is of God:" 1John 4:2 -- to mean that whatsoever spirit confesses that "He [that] shall save His people is come in the flesh is of God?"
IOW, Jesus was so named because "He shall save His people" (Israel), Mt 1:21.
Now many denominations have given up on Christ saving "His people."
They believe "replacement theology" or amil/postmil -- they see no future for Jesus people.
You know, the parameters John gives in 1John 4 look pretty "loose" to me.
1) Jesus Christ is come in the flesh (4:1) and 2) "hear us" (4:6).
This is how we are to discern false teachers?
==Christ will save His people. However His people are not just Jews, His people are all of those the Father has given to Him to save (Jn 17:2,24). So I would not agree that Matthew 1:21 is talking about the future of Israel. Are there passages that talk about Israel's future? Certainly. Paul is clear, for example, in Romans 11:11-32 that Israel has a future. God has not thrown away the Jewish people, He has not forgotten the promises He made to them. So I certainly do not hold to replacement theology and, since I am firmly pre-millennial, I am not a/post-millennial.
As for 1John 4:2, I don't understand how you are applying this to the Israel issue. John is responding to an error that was a problem in his day (a form of gnosticism). Anyone who denies that Jesus came, in a real body, is not a Christian teacher.