A 'proof' of God's existence - free will

[Belacqua]

...one of the attributes of said god is omnipotence.  If a creature has free will then god has incomplete power over it, therefore god is not omnipotent, therefore by the rules set up by the jewish, christian and muslim religions, that god cannot exist.

If you're reading the word "omnipotence" to mean that God controls in detail everything that happens, then, yes, it rules out free will. 

I'm not sure this is what Christians mean by "omnipotent," however. 

For a lot of Christians, it seems to mean "can do anything." In that case God could control everything in complete detail, but he could also choose not to. He could choose to let us make up our own minds. This is the standard view, I think. He chooses not to make us marionettes. 

As always, the naive Christian view is not the same as the theologians' view. Thomas Aquinas, for example, is clear that there are lots of things God can't do. He can't go against his nature, he can't do evil, he can't make a four-sided triangle, etc. etc. When Thomas says "omnipotent" he means something very different. 

In the Aristotelian/Thomist view, "omnipotence" refers to the fact that God is pure act without potency. In more common terms, he is entirely actualized with nothing left to develop or accomplish. The argument is that for potencies (potentialities) in the temporal world to actualize, there must be something already actualized which causes that actualization. And at the end of the chain there has to be one thing in which all potencies are complete -- actus purus. 

I'm aware that most Christians don't think of it this way, but Thomas Aquinas was a Christian. And this is the philosophy section of the forum, after all. 

I don't understand why christians constantly make out that we have free will, it breaks their whole religion.

It breaks the view of people who define the word "omnipotent" in the way that you do, but I'm not aware of any Christian who does.