The Christian God is defined as a personal being who knows everything. According to Christians, personal beings have free will.
In order to have free will, you must have more than one option, each of which is avoidable. This means that before you make a choice, there must be a state of uncertainty during a period of potential: you cannot know the future. Even if you think you can predict your decision, if you claim to have free will, you must admit the potential (if not the desire) to change your mind before the decision is final.
A being who knows everything can have no "state of uncertainty." It knows its choices in advance. This means that it has no potential to avoid its choices, and therefore lacks free will. Since a being that lacks free will is not a personal being, a personal being who knows everything cannot exist.
Therefore, the Christian God does not exist.