Can you disbelieve what you believe? Nonsensical question. You can change your beliefs or choose to act contrary to what you believe. A belief may not be well founded or strongly held, or may even be in error if that's what you're asking.
It's not a nonsensical question in the face of the claim that it is sinful to not believe/reject God/free will issue.
If a belief is a process of conviction, then it is not a willed process, it is the information we acquire that either convinces us or not. Whether we find something convincing also depends on many factors, education, mental capacity to understand, hopes, fears, desires, biases, flaws...none of which include 'free will'
So if a God, assuming existence for the sake of argument, sees sin in a lack of conviction in the absence of evidence, and condemns people on that basis...there is something very seriously wrong.
