Good and evil.

Allah differs from God in many ways. 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 have pointed out some of the cornerstones upon which Christianity is based, and how Islam differs from it. This is by no means an exhaustive list, but it is enough to say that Islam is no closer to Christianity than the infamous Gnosticism...