I think a lot of people in this thread are confusing two very important distinctions. So let me try to explain this to everyone.
An atheist and a Christian can both believe in Jesus. The atheist believes that Jesus was nothing more than a historical figure, while the Christian believes Jesus is the Son of God. They both are referring to the SAME Jesus, but their beliefs about who Jesus is are radically different.
The same thing goes for God. Christians, Jews, and Muslims all believe in the God of Abraham. It's the same God. But from that point on, their beliefs about who that God is are radically different.
I mean, Muslims believe in Jesus. They believe he was a great prophet of Allah. Is this a different Jesus that they are referring to than the Jesus Christians believe in? The answer is yes and no. It is the same historical figure, but both religions have extremely different views on who that historical figure is. The same goes for God.