I can think of myself in my mind as existing. I can think of you as existing at least in my mind. I can think of the collective of all great people as existing in my mind. I go further and assume that I can also assume an entity greater than all this. The greatest I can imagine in my mind. If the greatest such being is possible for me to imagine in my mind, then it must be true that this entity also exists in reality outside of my mind. For if he did not then there must be entities greater than himself.
Therefore God exits. Devious argument, at best.