Reason No. 1 - god is Silent
If God wants something from me, he would tell me. He wouldn't leave someone else to do this, as if an infinite being were short on time. And he would certainly not leave fallible, sinful humans to deliver an endless plethora of confused and contradictory messages. God would deliver the message himself, directly, to each and every one of us, and with such clarity as the most brilliant being in the universe could accomplish. We would all hear him out and shout "Eureka!" So obvious and well-demonstrated would his message be. It would be spoken to each of us in exactly those terms we would understand. And we would all agree on what that message was. Even if we rejected it, we would all at least admit to each other, "Yes, that's what this God fellow told me."
Excuses don't fly. The Christian proposes that a supremely powerful being exists who wants us to set things right, and therefore doesn't want us to get things even more wrong. This is an intelligible hypothesis, which predicts there should be no more confusion about which religion or doctrine is true than there is about the fundamentals of medicine, engineering, physics, chemistry, or even meteorology. It should be indisputably clear what God wants us to do, and what he doesn't want us to do. Any disputes that might still arise about that would be as easily and decisively resolved as any dispute between two doctors, chemists, or engineers as to the right course to follow in curing a patient, identifying a chemical, or designing a bridge. Yet this is not what we observe. Instead, we observe exactly the opposite: unresolvable disagreement and confusion. That is clearly a failed prediction. A failed prediction means a false theory. Therefore, Christianity is false.
Typically, Christians try to make excuses for God that protect our free will. Either the human will is more powerful than the will of God, and therefore can actually block his words from being heard despite all his best and mighty efforts, or God cares more about our free choice not to hear him than about saving our souls, and so God himself "chooses" to be silent. Of course, there is no independent evidence of either this remarkable human power to thwart God, or this peculiar desire in God, and so this is a completely "ad hoc" theory: something just "made up" out of thin air in order to rescue the actual theory that continually fails to fit the evidence. But for reasons I'll explore later, such "added elements" are never worthy of belief unless independently confirmed: you have to know they are true. You can't just "claim" they are true. Truth is not invented. It can only be discovered. Otherwise, Christianity is just a hypothesis that has yet to find sufficient confirmation in actual evidence.
Be that as it may. Though "maybe, therefore probably" is not a logical way to arrive at any belief, let's assume the Christian can somehow "prove" (with objective evidence everyone can agree is relevant and true) that we have this power or God has this desire. Even on that presumption, there are unsolvable problems with this "additional" hypothesis. Right from the start, it fails to explain why believers disagree. The fact that believers can't agree on the content of God's message or desires also refutes the theory that he wants us to be clear on these things. This failed prediction cannot be explained away by any appeal to free will
So this theory doesn't work. It fails to predict what we actually observe. But even considering atheists like me, this "ad hoc" excuse still fails to save Christianity from the evidence. When I doubted the Big Bang theory, I voiced the reasons for my doubts but continued to pursue the evidence, frequently speaking with several physicists who were "believers." Eventually, they presented all the logic and evidence in terms I understood, and I realized I was wrong: the Big Bang theory is well-supported by the evidence and is at present the best explanation of all the facts by far. Did these physicists violate my free will? Certainly not. I chose to pursue the truth and hear them out. So, too, I and countless others have chosen to give God a fair hearing
Even when we might actually credit free will with resisting God's voice
I know this for a fact. Back in my days as a flight-deck firefighter, when our ship's helicopter was on rescue missions, we had to stand around in our gear in case of a crash. There was usually very little to do, so we told stories. One I heard was about a rescue swimmer. She had to pull a family out of the water from a capsized boat, but by the time the chopper got there, it appeared everyone had drowned except the mother, who was for that reason shedding her life vest and trying to drown herself. The swimmer dove in to rescue her, but she kicked and screamed and yelled to let her die. She even gave the swimmer a whopping black eye. But the swimmer said to hell with that, I'm bringing you in! And she did, enduring her curses and blows all the way.
Later, it turned out that one of the victim's children, her daughter, had survived. She had drifted pretty far from the wreck, but the rescue team pulled her out, and the woman who had beaten the crap out of her rescuer apologized and thanked her for saving her against her will. Everyone in my group agreed the rescue swimmer had done the right thing, and we all would have done the same
So we can be certain God would make sure he told everyone, directly, what his message was. Everyone would then know what God had told them. They can still reject it all they want, and God can leave them alone. But there would never be, in any possible Christian universe, any confusion or doubt as to what God's message was. And if we had questions, God himself would answer them
Despite this conclusion, Christians still try to hold on to their faith with this nonsense about free will
There can't be any excuse for God, either. There are always disagreements, and there are always people who don't follow what they are told or what they know to be true. But that doesn't matter. Chemists all agree on the fundamental facts of chemistry. Doctors all agree on the fundamental facts of medicine. Engineers all agree on the fundamental facts of engineering. So why can't all humans agree on the fundamental facts of salvation? There is no more reason that they should be confused or in the dark about this than that chemists, doctors, and engineers should be confused or in the dark.
The logically inevitable fact is, if the Christian God existed, we would all hear from God himself the same message of salvation, and we would all hear, straight from God, all the same answers to all the same questions. The Chinese would have heard it. The Native Americans would have heard it. Everyone today, everywhere on Earth, would be hearing it, and their records would show everyone else in history had heard it, too. Sure, maybe some of us would still balk or reject that message. But we would still have the information. Because the only way to make an informed choice is to have the required information. So a God who wanted us to make an informed choice would give us all the information we needed, and not entrust fallible, sinful, contradictory agents to convey a confused mess of ambiguous, poorly supported claims. Therefore, the fact that God hasn't spoken to us directly, and hasn't given us all the same, clear message, and the same, clear answers, is enough to prove Christianity false.
Just look at what Christians are saying. They routinely claim that God is your father and best friend. Yet if that were true, we would observe all the same behaviors from God that we observe from our fathers and friends. But we don't observe this. Therefore, there is no God who is our father or our friend. The logic of this is truly unassailable, and no "free will" excuse can escape it. For my father and friends aren't violating my free will when they speak to me, help me, give me advice, and answer my questions. Therefore, God would not violate my free will if he did so. He must be able to do at least as much as they do, even if for some reason he couldn't do more. But God doesn't do anything at all. He doesn't talk to, teach, help, or comfort us, unlike my real father and my real friends. God doesn't tell us when we hold a mistaken belief that shall hurt us. But my father does, and my friends do. Therefore, no God exists who is even remotely like my father or my friends, or anyone at all who loves me. Therefore, Christianity is false.
The conclusion is inescapable. If Christianity were true, then the Gospel would have been preached to each and every one of us directly, and correctly, by God