If you can only believe what your reasoning compels you to, you are not a free will agent, but rather only a tool of that system of thought.
If you argue that free will doesn't exist, then you are arguing that your argument is nothing more than the wind blowing through the trees - the sound made by the inevitable sequences of physics. If it seems to make sense to you, or to others, it is only because physics commands us to think that - which would be the same reason for everyone - fundamentalists, scientists, logicians, the spiritual, and the cliinically insane.
Only if free will exists do debates like these have any coherent basis for meaning. We're all assuming everyone has the free will to understand and respond in a way that isn't coerced by physics. We're all assuming we have free will in every such debate or discussion.
So, free will is a necessary component of all coherent, meaningful discussions. For one to have free will, one's will, or intent, must be free from coercion by physics. To be free of coercion by physics, free will must be beyond physics and more powerful than physics. It must be empowered by something more powerful than, or unencumbered by, the operating rules of the physical universe, to override the impulses of cause and effect.
Therefore, every such discussion is an implicit admission that god (our putative source of free will) exists.