Good and evil.

Allah differs greatly from God. Here are a few key differences:
1) God is a Trinity, while Allah is not.
2) According to Christianity, people have free will; according to Islam, they do not; Islam is a fatalistic religion.
3) God became incarnate to atone for the sins of people; Allah did nothing of the sort.

Essentially, I've pointed out some of the fundamental tenets of Christianity and how Islam differs from them. There are many more points, but these are enough to say that Islam is no closer to Christianity than the infamous Gnosticism...

I disagree! Islam is just as much a "child of Christianity" as Christianity is a "child of Judaism." Everyone knows Muhammad, the prophet of Allah, but in Islam, figures like Jesus and Moses are also revered, albeit to a lesser extent. This was especially common before the Crusades. Afterwards, some animosity arose between these religions. There's also a theory that a new prophet appears approximately every 600 years, bringing with them a new religion. While this is a pseudo-historical theory, consider:
Moses
Buddha
Jesus
Muhammad
---I don't know who, maybe you can suggest---
Karl Marx

The time intervals between them are roughly 600 years. Regarding the fifth, I think it might be Martin Luther, and I'm not entirely sure about the last one. Furthermore, Hinduism appeared 600 years before Moses. Is this some kind of pattern?

BUT I WANT TO DRAW YOUR ATTENTION TO THE TOPIC TITLE! THIS ISN'T A HISTORY OF RELIGIONS! IT'S ABOUT "GOOD AND EVIL."
Therefore, whoever wrote that everyone sets their own rules should read the last chapters of "Crime and Punishment," where Raskolnikov dreams about the future. I generally agree with Silvanis about the framework of morality.