Jesus said that eternal life is knowing God. So, it's not about what you know, nor even how "good" you are. It's about WHOM you know. You can think about Heaven as an eternal victory party, with tickets provided for everyone who personally knows the host. No one can pay enough money, build enough churches, help enough people, or avoid enough bad deeds to get in to the party. It's all inconsequential when it comes to getting in the door. Only one thing matters. Is your name on the guest list?
"If I committed murder, or even genocide for that matter, but sincerely and graciously asked for God's forgiveness, would I get it?" Sincerely asking for Gods forgiveness means that you believe there is a God, and that you believe that He rewards those who seek Him. That is the bottom line entry-level requirement to be in right standing with God. (Hebrews 11:6)
...I then pointed out that for the sole reason that I am an atheist, I cannot be forgiven. You say that as if being an atheist is your identity rather than your belief system. You can totally be forgiven if you stop being an atheist and believe in God. Unbelief is a sin, not a part of your identity.
According to this, Atheism is a worse crime than murder or even genocide. You are right! But even if a small part of you believes or wants to believe, you can pray the prayer of Mark 9:24 - "I do believe; help my unbelief!" I once thought I had lost my ability to believe, and I prayed that prayer. It was years later, but God brought me back to belief in answer to that prayer.
Well, first of all, do you agree/accept this claim that you can be forgiven for murder/genocide, and if not, then why not? Absolutely.
If you agree with the claim, do you believe this is fair? why/why not? I believe "fair" is a human, and therefore flawed, concept. It is a fruit of the tree of the knowledge of good and evil, and by that I mean, it springs from the notion that humans decide on morality. That is not a Christian notion. Morality for the Christian (and the Jew, by the way) springs from the character of God. The godly counterpart to fairness is justice. According to justice, God determines what is moral and right and good, and I do believe that the belief requirement is just, because God requires it, and He is always right. God (and not some platonic ideal or human idea of fairness) is the north star of goodness.
From a neutral or Atheistic stand point, can you see why this could be very confronting? Yes, and it's supposed to be. "Just as it is written, 'Behold, I lay in Zion a stone of stumbling and a rock of offense, and he who believes in Him will not be disappointed.'"
What I mean by this is that for someone like me (an atheist) I see this kind of thing as terribly immoral and unjust. As soon as you stop seeing yourself as an arbiter of what is moral and just, and look to God, you will be free of the pride that barred Lucifer from Heaven.
blessings
tal