Does man have free will, and what are the implications of the answer to that question? Ironically, those who would answer "no" often fall on opposite ends of the piety spectrum. It takes a lot of love for religion to be a Calvinist. Generally speaking, Calvinists are very passionate about their doctrines, but unfortunately, many of them (not all) reject the existence of free will.
Liberals also have a very diminished view of free will. These people tend not to be so pious, often casting doubt upon their own salvation, showing minimal if any Christian fruits. We often hear such people asking things like why would God "make" people gay if He was also going to command against homosexuality?
Both of these options are extremely problematic. If free will exists, the one responsible for sin is the sinner. When we sin, God takes no blame for that. We choose it. But what if our will is subject entirely to God? What if God does not give us the ability to choose whether or not we rebel against Him? Then we have the problem of sin still existing. If free will does not exist, there can only be one source of sin: God Himself. And yet, if God is the source of sin, God is the only sinner that exists!
I hope I do not need to explain why this philosophy is unsustainable. For God to be a sinner is the literal antithesis of Christianity. For this reason, James rightly tells us that when we are tempted, it is flat out wrong to say "God is tempting me", because God cannot be tempted by evil, neither does He tempt anyone. We don't serve a slimy God who tricks us into sin, or forces us to sin, or is in any way responsible for our sin. We serve a Holy God, who is not only opposed to sin, but especially for the Christian, promises that we will never encounter more temptation than we can handle, and that with such temptation, He will provide a way out (1 Corinthians 10:13). Therefore, every time a Christian sins, they either did so because they had a choice and made the wrong one, or because they didn't have a choice, and God lied.
We see, then, that free will is more than just a basic response to the "problem" of evil argument used by atheists. It is also an essential principle that has implications about the truth value of the Bible, and the character of our God. Only if human beings have free will can we be responsible for our own sin without implying that God is evil, and is punishing us for works He made us do.