How do you do that without confusing them, and how do you get the knowledge to do it, and how do you prepare them so they are able to believe in a exclusive religion such as Islam or Christianity if they so choose?
I don't. Not my job to try to lead them down a particular path, or 'prepare' them for anything, or promote any particular faith or type of faith above others.
We talk about the fact that different people have different ideas about the world and how it was made, we talk about some of the different things people believe and some of the different things they celebrate, we talk about science and natural wonder. He knows that there are different sects of Christianity, and doesn't struggle with that.
In an ideal world, he'd have access to everything on religion presented to him fairly evenly, from a secular base. I do not live in that world, so he gets Christianity promoted above other religions. To spare him confusion and dissonance between what school says and what his parents say, we are currently avoiding telling him that I don't think God is real. But I am careful to let him know that different people believe different things, and that some people don't believe in any gods, and that's fine too.
If he finds any faith particularly compelling I will support him in exploring it. I personally hope he doesn't, of course, but I don't see it as my role to try to make him think as I do. That is a heavy burden for someone to bear.
That's a long post. In summary - it is more important to me that my children be able to think critically, embrace difference and diversity, and be secure in making their own choices based on a strong ethical foundation, than that they think like me.