The fundamental belief that I base my morals on: harming people is a bad thing, and should be avoided where possible.
Stipulation to the above: Letting someone be harmed when you could easily avoid it is no different than harming them yourself. (If I see a child drowning, and sit there and watch rather than try to save them, I am just as responsible for their death as I would be if I had drowned them with my own hands rather than just watching).
Conclusion based on the above: When I have the power to prevent harm, I must use it to prevent harm. To do otherwise would be immoral.
Seems like a fairly self-explanatory utilitarian moral stance, right?
Well, where it gets tricky is when you add to that the belief in a higher power.
See, by that metric I just laid out god is not a good person.
Not only is he responsible for many murders and other forms of harm that he directly caused people in the bible, but the fact that he has an unlimited amount of power means that any amount of harm could be avoided if he wanted it, and doing so wouldn't even require him to break a sweat. Not only that but he is apparently all knowing, so he can't even claim the defense of not seeing the harm or not knowing the possible consequences of his actions.
So every bad thing that has ever happened on earth can be directly attributed to gods negligence, and as I believe negligence in preventing harm when you could easily stop it is the same thing as causing harm, God can thus be held morally accountable for all of those things.
The Holocaust happened because the nazis were monsters, but if god exists then it ALSO happened because God looked into the future before creating the world and said 'yup, the holocaust is totally a thing I want to happen, let me just create the world in such a way that it ensures that it does'. He saw the people dying, he heard their screams, and he did nothing.
The same goes for every other bad thing, every-time someone is raped god is sitting their watching, with the power to end it with a word, yet he chooses not to. Because it was what he wants to happened, because he made it happen, because he enjoys watching human beings suffer, and is at best apathetic to our pain.
Now depending on what type of christian you are, your view on hell will vary greatly. Many don't believe in it at all, or view it as mere separation from god rather than the fire and brimstone thing. (as that interpretation comes from Dante rather than the bible, which only mentions a lake of fire that god throws the rebellious angels into in Revelations), but in general most Christians believe in hell in some form, and it is generally accepted to be some form of eternal (or at least, very long and severe) punishment for those who fail to confess to their sins on earth.
This, is unacceptable.
Everything that has happened on earth is perhaps forgivable. It makes god a monster, and would certainly take quite a while to forgive, but if God dumped everyone that died in heaven then it might be possible.
But he doesn't. Heaven is reserved for those few people that god likes a lot, for everyone else he has torture.
Now, to me, even the suffering of an evil man is undesirable if it can be avoided without causing further damages, but some people will disagree with me on that. But it doesn't matter since god doesn't just dump evil people in hell, because christian doctrine is that everyone, even babies, are tainted by the original sin, so anyone who does not confess their sins to the lord before they die goes to hell.
Do you realize how insane that is? do you know the sheer number of good people who lived and died in cultures that simply never had any contact with Christianity? do you know how many children would fit that definition?
Above and beyond anything that happens on earth, how could I ever accept any kind of god who would torture a man simply for being raised Hindu? Why should I forgive the sins of a god that would torture someone who dedicated their life to protecting children just because they were an atheist?
The answer is I shouldn't. Your god is evil.
And were I to worship him in spite of that, I would be supporting that evil.
The christian god as depicted in the bible is petty. The dude creates an entire universe, but kills people for whining, and apparently cares what human beings do and think. God wants people to worship him, he wants people to tell him that what he does is good.
But it's not. Letting children die is not good. Giving innocent men cancer is not good. Letting people starve is not good. Torture is not good. Murder is not good. Rape is not good.
If God is as depicted in Christianity then he commits many horrible, entirely unethical acts. He allows things to happen that any moral person would not. AND he CAN be influenced by human actions, as we see him do in the bible.
If those that is true, then it is a moral imperative that we not be Christians. If there is even the tiniest possibility that god can be influenced to not torture people forever then I must pursue it.
In all likelihood it is futile. God will never change and the only thing I will have accomplished is ensuring my own damnation. But I would rather suffer in hell with a clear consciousness that live heaven with a bloody one.
So I am going to live my life, I am going to be the most moral person I can be, I will avoid harming people and keep people from being harmed where possible. And then when I die if god turns out to be real I will walk up to him and spit in his face.
From there either hell exists and I will be sent to it, or it doesn't and I will go to heaven regardless.
But there is no version of me that could permit myself to be in heaven while even a single person was in hell.