The Bible.

Free will inherenly includes the facility to do the wrong thing. Always avoiding the wrong thing would not be free will. Since free will is the only thing that provides the facility to sin, then free will itself must be wrong.

I suppose you could argue that free will means the ability to choose indiscriminately amongst multiple non-sin options, but that argument doesn't seem to be supported by what you're quoting.

In addition, Calvinists would claim that you're out-and-out wrong, and I don't think anyone could make a reasonable argument that they're not Christian.