1 Corinthians 10:13 is a favorite verse for many Christians, as it delivers a very comforting promise: "No temptation has overtaken you except such as is common to man; but God is faithful, who will not allow you to be tempted beyond what you are able, but with the temptation will also make the way of escape, that you may be able to bear it." This beautiful promise, however, carries a nasty sting. On the one hand, it means I will never, as a Christian, face a temptation that God will not help me overcome. But it also means every time I do not overcome temptation, which, as Christians, we have to admit happens on a daily basis, it is 100% my fault. I could have avoided sin, but I chose not to.
This is indisputable proof that we have free will. If it is not only God's will that we avoid sin, but also His promise that we always can, then the only will that could possibly allow me to give in to the temptation I was able to escape is my own. Free will, in spite of Calvinist protestations, is therefore proven by a single verse.
But a Calvinist might suggest this only applies to Christians. We have free will, they say, but the unbeliever does not. To that, I say James disagrees. In James 1:13-14, we read "Let no one say when he is tempted, “I am tempted by God”; for God cannot be tempted by evil, nor does He Himself tempt anyone. But each one is tempted when he is drawn away by his own desires and enticed." Now, if God cannot be tempted by evil, and therefore does not tempt anyone, then any and all temptation must be because the one being tempted is drawn away by his own will, not by God's.
But let's pretend James never wrote his epistle, and the only Scriptures we have proving free will only prove Christians have free will. That means the Holy Spirit, while He increases our ability not to sin against God, also gives us a greater capacity to choose to rebel against Him!
This doesn't sound like God in any theology I've ever heard, least of all Calvinism. In fact, ironically, it sounds closer to a branch of Arminianism, which teaches that while man is totally depraved, God partially regenerates the entire human race to the point of being able to choose Him.
It is repugnant to me that the God who cannot deny Himself (2 Timothy 2:13), and is able to prevent His children from stumbling (Jude 1:24), would approach a people who have no choice but to rebel against Him, only to cause them to actively choose to rebel against Him. In light of Scripture, it makes far more sense to me that mankind has free will from the day we are born, that we fall into temptation, and that both prior to and after conversion, we freely choose to disobey God. It's just that, for the unbeliever, they choose the one sin God cannot forgive: Rejecting the command to repent and believe in Christ. All other interpretations seem to conflict with Scripture.