If you take this line at its face value "God is not God apart from Jesus. It is pointless to try to define the God of Abraham, Isaac and Jacob apart from Jesus Christ."
It leaves Jews in the same boat as the Muslims. I'm not willing to do that.
Also would Abraham, Isaac and Jacob have known Jesus?
Jesus was known in vague ways, through the early prophecies, the Seed of the woman (Eve) who would eventually crush the serpent’s head. The full flowering of messianic prophecy would come later through the prophets of Israel, David, Isaiah, and others. Jesus repeatedly points back to the Old Testament prophecies as teaching about Himself.
The difference is, when Jesus actually appeared, all those superstar rabbinic scholars who had build up wrong expectations about Messiah could not accept him as the humble suffering servant. Their formal belief in the prophecies did them no good when they met the real deal.
But it is Jesus Who laid down the necessity of believing in Him. So if a person is presented with the claims of Jesus to be Messiah,, and they reject those claims, that demonstrates that they have not yet received the gift of faith. Jesus plainly said all that the Father gives to Him will come to Him.
So this is serious business. Jesus is the founder of our faith, and He is God in the flesh. If He says we really need to believe in Him, then it must be true, no matter how unpleasant the implications might be for us. Otherwise, if we reject what He says about this, how can we claim to be His disciples? It’s pretty much all or nothing. I have loved ones too that I anguish about. But I can’t alter the truth.
Peace,
SR