[You somehow managed to misquote what I wrote. The words "many will come from east and west, and sit down
with Abraham, Isaac, and Jacob in the kingdom of heaven" was what you wrote, not me.]
A doesn't equal B, but if you are a Christian then you are also heirs according to the promise - the promise that in Abraham's seed the nations would be blessed. Abraham was not promised that he personally would be involved in blessing the nations, the promise to Abraham was that his seed, that is Christ, would bless the nations. As Paul wrote, if you are a Christian then you are considered to be part of Abraham's seed (Gal 3:29) - part of Christ, the body of Christ, with Jesus the head of Christ (anointed). Christ was anointed to be the redeemer and saviour of the world, not Abraham. Abraham was promised land, that he would be the father of many nations and that all nations would be blessed by his seed (Christ).
Paul wrote, Galations 3:16 (KJV):
(16) Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.
The promise was to Abraham, but also to Abraham's seed - Christ, head and body. Galations 3:27 :
KJV - (27) For as many of you as have been baptized into Christ have put on Christ.
LSV - (27) Indeed, all of you who were baptized into the Messiah have clothed yourselves with the Messiah.
Barnes Notes says:
Have put on Christ - That is, they have put on his sentiments, opinions, characteristic traits, etc., as a man clothes himself. This language was common among the ancient writers.
To put on Christ means to be conformed to Christ's character, to become like Christ, to be one with Christ, which is something that Abraham and the Old Testament times people could not do, because Jesus had not yet come to earth to reveal what he (and God) were like.
What does "A + B = C" mean? Communicate clearly what you are trying to say! The passage you are quoting (Mark 12:26-27) is Jesus answering a specific question about whose wife a woman would be in the resurrection if she had married multiple men before she died. Jesus then commented on the resurrection, because he was speaking to the Sadduces who didn't believe in the resurrection of the dead. Jesus quoted Exodus 3:6 to make the point that the dead are still in existence, that is their souls are still alive but in a dormant sleeping state (Daniel 12:2 - "Many of those who
sleep in the dust of the earth will awake"), and that God will awake them in the resurrection and give them a new body. As Barnes Notes says:
As touching ... - That is, in proof that the dead are raised.
The passage which he quotes is recorded in Exo 3:6, Exo 3:15, This was at the burning bush (Mark and Luke). Abraham, Isaac, and Jacob had been dead for a long time when Moses spoke this - Abraham for 329 years, Isaac for 224 years, and Jacob for 198 years - yet God spake then as being still “their God.” They must, therefore, be still somewhere living, for God is not the God of the dead; that is, it is absurd to say that God rules over those who are “extinct or annihilated,” but he is the God only of those who have an existence. Luke adds, “all live unto him.” That is, all the righteous dead, all of whom he can be properly called their God, live unto his glory. This passage does not prove directly that the dead “body” would be raised, but only by consequence. It proves that Abraham, Isaac, and Jacob had an existence then, or that their souls were alive. This the Sadducees denied Act 23:8, and this was the main point in dispute. If this was admitted - if there was a state of rewards and punishments - then it would easily follow that the bodies of the dead would be raised.
But again, what has that got to do with who has a part in the first resurrection rather than the second resurrection?
Yes, people could only visit Abraham if they and Abraham have been resurrected, obviously.
The resurrection is only possible because of Jesus' sacrifice of his life, which is part of his blessing to all nations and makes it possible that everyone can be resurrrected. But the resurrection is not dependent on God having promised Abraham that the Messiah would bless all nations. God simply informed Abraham that all nations would be blessed by one of Abraham's descendants.
No, our being heirs with Jesus does not depend on Jesus being God (for he isn't!), nor does it depend on Jesus being with God in the beginning of His creating the universe and all life on earth. Even so, it has nothing to do with determining who can be heirs with Jesus and have a part in the first resurrection.