Was it at this point where Christian and muslim starts it's division?

I believe Mohammad was influenced by both Gnostic and pagan tradition when he developed his ideas. It is important to Muslims to disseminate the meaning of their tradition by pointing to specific biblical texts that they claim supports their view that Mohammad was the last in a line of prophets sent by god to reveal his message to mankind.
Mohammad being the last in a line of prophets carries with it the idea that God has said all that needs to be said concerning his will and so therefore no more needs to be revealed as this final revelation is contained in the Koran.
There is more to this controversy. Muslims do not believe christ to be divine as he was only another in a line of prophets who failed in his mission. In addition, Muslims point out that any attempt to show that Christ is divine cannot be taken seriously because as they point out the new testament cannon is composed of texts that are not genuine but rather forgeries and as such are untrustworthy.

I don't think so. It started because the muslim's believe that Ishmael is Abram's first born, therefore the rightful heir of the promise. Christian's know that it was Isaac that the promise comes through not Ishmael. Read Genesis, chapters 15 - 22.

I am not wrong about my point as you say. Lets be clear about the question at hand. What is the main dividing point between Christian and Muslim theology? It has to do with who the messiah is. You are right about Ishmael being born first and therefore he is the father of the Arab lineage. It however, does not follow that Muslims can claim anything more than that. To suppose that the old testament points to Mohammad as the fulfillment of that lineage from Ishmael has credence only in the lineage not in the prophecy as the coming messianic figure. I am well aware of the firstborn issues you are referring to. I am speaking about the division between theological presuppositions regarding the messiah. Christians understand that Jesus was the fulfillment of Isaiah 61. Muslims do not and therefore the dividing issue is between Christian and Muslim theology on this particular claim. Weather or not the lineage from Isaac or Ishmael is argued it makes no difference. The offending point is twofold:
1> That Christ is divine and therefore God.
2>That Christ is the fulfillment of Isaiah chapter 61 as to who the Messianic figure is. See Luke 4:21 >and he began by saying to them, "Today this scripture is fulfilled in your hearing." When Christians point out that Luke's passage is pointing to Jesus as the fulfillment of this text as the Messiah they counter this claim by saying that Isaiah 61 is a corrupted text (not genuine) and as such it follows that Luke’s account only continues to promulgate the Christian heresy of Christ. Therefore these are not genuine texts but forgeries of original texts that have been lost in history.

What is curious about this and other texts that Muslims point to concerning Mohammad is their claim that in the very same book that speaks of Mohammad as the coming messiah (Isaiah chapter 53 ) the text somehow changes from being genuine once it gets to chapter 61 concerning Christ , to being a corrupted text!
In addition to their claims to Mohammad fulfilling old testament passages Muslim’s claim that Mohammad is set apart from all other prophets as the fulfillment of the incarnation of the Holy Spirit as alluded to by Jesus when He spoke of the coming of the comforter who Muslims interpret as Mohammad himself. Here they continue as they do in the old testament by calling particular texts genuine while saying that other new testament texts are corrupt versions of original texts lost in history. By separating specific texts to show Mohammad fulfilling the prophecies and leaving out texts that point to Christ they continue to build this heresy.
The final version one ends up with concerning God’s revelation to the world is very different from the Christian tradition in which God has provided Mohammad with His final word , the Koran. Although Muslims hold to certain parts of the old and new testament and use these to texts to validate their theology they claim that these texts pale in comparison to the Koran in that this document is superior to and unlike the old and new testament manuscripts that we possess today which is completely free from error in both its doctrine and its text.

Extremely well said!

According to Numbers 12:6, if there is a prophet among us the Lord will make Himself known to him, and talk to him by means of dreams or visions. Abraham was a prophet. When he left Ur of the Caldeans and headed on the direction of Canaan, he dwelled in BeerSheva. By observing how the Canaanites loved their gods so much that they would offer their firstborn son in a burn sacrifice to their gods, Abraham would get deeply impressed.

Then, one day, Abraham had a vision in which he would take his son Isaac to offer him in a burn sacrifice to God in the Mount Moriah. At the climax of the sacrifice God would interfere and stop him by saying that Abraham had been successful in the test that he indeed loved God. Then, a lamb that had been provided for the moment, was at Abraham's reach to be offered instead of Isaac.

The point though was that God's People from the seed of Abraham did not have to demonstrate his love for God as a pagan. No human sacrifices were permitted. And that no one was to die for another but that everyone was supposed to die for his or her own iniquity. (Jer. 31:30)

The bottom line here is that the attempt to sacrifice Isaac
was not literal but an event that occurred during a vision that Abraham had.