So I take it you're not a determinist? That's a whole conversation that's separate from this, but you should know there are a lot of secular people who don't believe in free will (e.g having a will independent of any casual relationships to physical reality). Secular people are generally deterministic, we believe that wills exist within physical reality, and that they exist in the same cause/effect relationship as everything else.

With enough information of the present, you could know everything a human will do in their lifetime, there's no will that exists outside of reality that is influencing reality (no will that is "free"). Instead, will is entirely casually linked, like everything else.

Put another way, you're guaranteed to get the same result every time you put a human in exactly the same situation. Even if there is true chaos in the universe (e.g pure randomness) that's a different situation every time you get a different random result.