A good working definition of “free will” is, “The ability of a creature to choose for itself on the basis of subjective values.”
When speaking of free will, we generally focus on choice, but our choices are determined by what we value, because we will always choose what we consider to be the most valuable option. Any conception of free will that does not allow the creature to set its own subjective and ever-changing hierarchy of values will ultimately end in determinism.
(Note: To say that decisions are based on subjective values is not a relativistic objection to the idea of objective value. Objective values exist, but we make our decisions based on subjective values, i.e. the values we actually personally assign to a given option. The existence of sin testifies to the human ability to act on the basis of subjective values that do not reflective objective reality.)