One fact that completely boggles my mind is that in the modern day, with all our great educational institutions, with our ridiculously high literacy rates, with our great access to both hard copy books and the entirety of the internet, there are still many people, particularly within the atheist community, who will claim that Judaism, Christianity, and Islam, are all the same religion. After all, allegedly, they are all Abrahamic.
There are several ways to disprove this assertion, not least of which being the Qur'an quite explicitly says not to take Jews or Christians as friends, because we are friends of each other, and that if a Muslim takes one of us as friends, they become one of us (Qur'an 5:52). But perhaps the easiest way to prove this is to simply go to the Christian scriptures.
A very important verse in the Bible is Jude 1:3. Here, we not only learn that the Christian faith has been delivered "once for all", but that we, as Christians, are to contend for it. This immediately discounts any later alleged revelations (which also allows us to discount the dogmas of the Catholic Church, and many other pseudo-Christian religions). Now, was Islam in existence during this time? The answer is a very obvious no. Not only was Islam invented roughly 600 years later, and of course went through its own period of evolution, but when it did come, it essentially undid both the Jewish and Christian faiths.
Muhammad constantly criticised Christians and Jews, often for things no Christian or Jew has ever believed. Christians do not, for example, believe Mary is a god besides Allah (Qur'an 5:116), nor has any Jew ever claimed Ezra is the son of Allah (Qur'an 9:30). But even the bits Muhammad correctly understood that Jews or Christians believe, he attacked. Muhammad claimed that Jesus was never crucified, and to call Jesus the Son of God, much less acknowledge His divinity, is considered "shirk" (associating partners with Allah). To cut this story short, let's just point out that even though the Bible is significantly better preserved than any work of antiquity (including the Qur'an), modern Muslims insist the Bible has been corrupted. That is how much Islam contradicts Christianity.
What that ultimately means is that Islam, rather than being the same religion as Judeo Christianity, is an impostor and a parasite. No sound reasoning would compel Christians to accept Islam, or indeed any later "revelation", as a part of their religion.
Now, one might ask, is Christianity not an impostor and a parasite on Judaism by the same logic? There are very simple answers answers to this. First, there is the contrast of the "closing". As we have already discussed, the New Testament effectively ends with the words "contend for the faith that was delivered once for all to the saints". By contrast, the Old Testament is a book that demands a conclusion it never received. It is flooded with promises of a Messiah; a Messiah who would usher in a new covenant (e.g. Jeremiah 31:31). Christians aren't asking Jews to accept novel revelations, and certainly not ones contrary to what they already have. Rather, we're both using the revelations the Jews already have, and want them to hold to them too! The Messiah they were promised is the Messiah we all received, so much so that you can often read the Old Testament to a Jew and, without telling them anything, they will assume it's the New Testament. Isaiah 53 is especially effective at this.
Can Islam say the same? Absolutely not. As we have discussed, the New Testament has a very clear conclusion, whereas Islam, as we have already discussed, attacks both Judaism and Christianity. Is Jesus the Son of God? Christianity says yes, Islam says no. Did He die on a cross? Christianity says yes, Islam says no. Did He rise from the dead? No brainer, finish the logic yourself, Christianity and Islam cannot be the same religion. By contrast, the only way to separate Christianity and Judaism is to say that God not only failed to send the promised Messiah, but Jesus, a mere man, did a perfect job of looking so much like the Messiah God promised that He deceived a huge number of devout Jews into laying down their lives for His lie.
The logical conclusion, therefore, is that Christianity is the conclusion of Judaism, whereas Islam is just one more false religion. Everyone needs to repent, and confess Jesus as Lord, because indeed He is, and this is the only way to be forgiven for our sins.