Genesis 21:12 - Son of Isaac

 The claim is that when God promised Abraham not to concern himself with Ishmael with the words:

And God said to Abraham, "Don't make this [casting out of Hagar and Ishmael] bad in your eyes concerning the youth and your conjugal slave. Do all that you hear her voice say and do what Sarah says to do, because it is with Isaac that will be referred to as your "seed".

Again, the idea of "seed" is the genetic offspring that goes through the male line. 

This is neither messianic, nor is it prophecy. It is simply stating that the lineage of Isaac will be the predominate one. It is akin to saying "You will not marry a woman from the Moabites". 

It wasn't a prophecy. To claim it was is simply a way of buffing up the "prophecies fulfilled" count.

Not every word reportedly said by God is a prophecy. Some are simply rules to organize one's life around.

In a previous post about the "seed of Abraham" supposed prophecy, we saw that, even if you want it to be a prophecy, the lineage cited in Matthew 1:1 is for Joseph, and Jesus was not Joseph's seed, so he could not fulfill that.

In this "prophecy", Luke 3:23 and 3:34 are cited instead. Except that, like Matthew 1:1, this is again connected directly to Jesus's step dad, and not Jesus:

3:23 - Now Jesus himself was about thirty years old when he began his ministry. He was the son, so it was thought, of Joseph, the son of Heli

3:34 - the son of Jacob, the son of Isaac, the son of Abraham, the son of Terah, the son of Nahor 

Clearly this is the lineage of Joseph, and not of Jesus. So Jesus was not the son of Isaac.

There are those who want to claim that this is a lineage about Mary, who is never mentioned. And they ignore that both Matthew and Luke have male-only lineages because it was only though the male line that connected one to a specific ancestor, and to a specific tribe.

This made Jesus tribeless. 

Because Mary could not associate him with the tribe of Judah, as David was, and his step-father didn't count, Jesus could not have been a "son of Isaac".

Style: non-messianic, non-prophetic, not fulfilled by Jesus

Here's the meme used: