Why do some christians believe that their God and Allah are not the same deity?

I read this claim from a lot of christians online even heard this claim from my former classmate that the christian God is not Allah, but christianity and islam are both abrahamic religions, both believe in the same god of Abraham.

Some people are saying that they are different gods because in islam they believe that god is only one, with no father nor a son, in christianity you have the trinity, but to me its all just different point of views.

Its like I have a friend, lets call him John, and I describe John to person A. Then there is person B that describes John to person A, but describes him in a totally different way. That doesn’t mean that there are two John’s, it just means that person A has now two different views of John.