Alright...easy one...show me free will exists without resorting to a deity.
It seems to me that, from a purely materialist perspective, it is nearly impossible to come up with a theory of how free will would operate. The ability to do otherwise than physical reality (or lets say...outside of causation) would appear to have to be on an extremely micro level (we're talking nearly no mass) AND would somehow have to have access to a null point an empty point. Without access to a null point, you'd just be stretching causation back to a point where we don't understand it. (not outside of causation).
Personal responsibility and free will are nice, and one can argue useful illusions. However, they are just that.
I'd even have trouble, being a Christian, with a Christian argument that supports free will.
All truths may be subjective, but truths nevertheless.