Judaism (the religion of the Jewish people), Christianity, and Islam (the Muslim religion), are all
Abrahamic religions.
If two or more religions are monotheistic (believe that there is only one god) that does not necessarily mean that it is the same god. However, in the case of these three major religions, it is definitely the same god. They all agree on that.
Abraham was given the birth name, Abram. He was Jewish. God later renamed him "Abraham" which meant "father of nations".
Abraham made the first "covenant" between man and God. It is this covenant that is the basis of the Jewish people believing themselves to be "God's chosen people."
In the Jewish religion, it was foretold that a Messiah would be sent by God. A Messiah is a saviour or liberator of a people.
Jesus is said to have lived at a time when the Romans ruled Israel. The Jewish people therefore needed a "saviour or liberator". There were no doubt many people who would have tried to claim the title for themselves.
Jesus' teachings were of humility, forgiveness, and turning the other cheek, loving your enemy, and seeking salvation in the love of God.
Christians believe that a person who follows the teachings of Jesus is "saved" and"liberated" (in a spiritual sense) and therefore that Jesus was the Messiah.
Most of the Jews of the time were hoping for a rather more pragmatic approach from God: a Messiah who would lead them to safety and liberation in a more physical and practical sense — probably a war leader or hero.
This is where the first split occurred, with most of the Jews sticking to the Jewish religion and waiting for the Messiah, while a few Jews and non Jews ("gentiles") accepted that Jesus was the Messiah and moved towards what they considered to be spiritual salvation through following Jesus' teachings.
Today, there are some people who follow much of the Jewish religion, but believe that Jesus was the Messiah. These are "Messianic Jews". They would no doubt offer a more nuanced explanation than this short paragraph!
Islam (the Muslim religion) is founded on the teachings of Mohammed, who was born around 570 AD. He led an army at one stage in his career and in that sense he was a war leader. However, he is also believed to have been a prophet who received instructions directly from God.
All 3 religions — Judaism, Christianity and Islam — accept much of what is in the Old Testament (Jewish religious texts from before the time of Jesus. Specifically, they all believe that Abraham was a prophet of God.
- Abraham may have existed as an actual living person, but I am aware of no clear historical evidence of this. Alternatively, he could be a mythical figure to who several stories and legends have been attached.
- Jesus may well have existed as an actual living person.
- Mohammed certainly existed.
- Jews do not accept that Jesus was the Messiah.
- Christians believe that Jesus was not only a prophet, but that he was actually God's only son.
- Muslims believe that Jesus was a prophet, but not as great a prophet as Mohammed, and that he was most certainly not the son of God.
Belief or faith are very much a personal matter. However, religion is an organised phenomenon, designed by people for their own purposes which may be good or ill. As others have observed, when they all worship the same God, and agree on so much, it is surprising that there has been so much bloodshed. However, in my view, religion is not to blame for most of that. Religion has often been the excuse for, but seldom the primary cause of conflict.