By Dan Barker
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.
Some people deny that humans have free will; but all Christians claim that God himself, "in three persons," is a free personal agent, so the argument holds.
Others will object that God, being all-powerful, can change his mind. But if he does, then he did not know the future in the first place. If he truly knows the future, then the future is fixed and not even God can change it. If he changes his mind anyway, then his knowledge was limited. You can't have it both ways: no being can be omniscient and omnipotent at the same time.
A more subtle objection is that God "knows" what he is going to do because he always acts in accordance with his nature, which does not diminish his free agency. God might claim, for example, that he will not tell a lie tomorrow--because he always tells the truth. God could choose outside of his nature, but he never does.
But what does "nature of God" mean? To have a nature is to have limits. The "nature" that restricts humans is our physical environment and our genetics; but the "nature" of a supernatural being must be something else. It is inappropriate to say that the "nature" of a being without limits bears the same relationship to the topic of free will that human nature does.
Free will requires having more than one option, a desire to choose, freedom to choose (lack of obstacles), power to accomplish the choice (strength and aptitude), and the potential to avoid the option. "Strength and aptitude" puts a limit on what any person is "free" to do. No human has the free will to run a one-minute mile, without mechanical aid. We are free to try, but we will fail. All of our choices, and our desires as well, are limited by our nature; yet we can still claim free will (those of us who do) because we don't know our future choices.
If God always acts in accordance with his nature (whatever that means), then he still must have more than one viable option that does not contradict his nature if he is to claim free will. Otherwise, he is a slave to his nature, like a robot, and not a free personal agent.
What would the word "option" mean to a being who created all options?
Some say that "free will" with God does not mean what it means with humans. But how are we to understand this? What conditions of free will would a Christian scrap in order to craft a "free agency" for God? Multiple options? Desire? Freedom? Power? Potential to avoid?
Perhaps desire could be jettisoned. Desire implies a lack, and a perfect being should lack nothing. But it would be a very strange "person" with no needs or desires. Desire is what prompts a choice in the first place. It also contributes to assessing whether the decision was reasonable. Without desire, choices are willy-nilly, and not true decisions at all. Besides, the biblical god expressed many desires.
No objection saves the Christian God: he does not exist. Perhaps a more modest deity can be imagined: one that is not both personal and all-knowing, both all-knowing and all-powerful, both perfect and free. But until a god is defined coherently, and then proven to exist with evidence and sound reasoning, it is sensible not to think that such a being exists.