the problem of evil. These are bad reasons, I think, and based on the presupposition that God doesn't exist. Personally, the hardest thing for me was believing that God was all-powerful. Like, if God were portrayed as good, but weak, struggling against an evil god and just doing the best he could to make a just universe and make his existence known, I probably would never have left the faith.

This puzzled me, since it sounds a lot like the problem of evil. I take it you were describing the argument you lay out at the link?

For completeness - since I'm about to bash Christianity - I should note that Paul does not write like he has even an imagined revelation on the subject of Hell. He writes as if people in the Roman Empire often talked about everyone going to Hades when they died, and therefore he could count on people receiving as "good news" the claim that belief in Jesus would definitely send you to Heaven. (Later, the Gospels implied that your actions could send you to Heaven or Hell regardless of what you believed. Early Christians might have split the difference by reserving baptism for those they saw as living a 'Christian' life.) Clearly one can be a Christian in Paul's sense without believing in Hell.

Who was the historical Jesus?

We don't know. I have some qualms about Richard Carrier's argument (eg in On the historicity of Jesus: Why we might have reason for doubt). But plugging different numbers into his calculations, I come out with no more than a 54% chance Jesus even existed. We can't answer every factual question; some information is almost certainly lost to us forever.

As a history source, why is the Bible unreliable?

This one seems fundamental enough that if people insist on the truth of miracles - and reports that you can move mountains if you have faith the size of a mustard seed - I don't know what to tell them. But besides directing people to mainstream scholarship (which by the way places the date of Mark after the destruction of the Temple), I can note that Mark inter-cuts the story of the fig tree with Jesus expelling the money-lenders from the Temple. The tree seems like a straightforward metaphor. Then we have later Gospels openly changing the narrative for their own purposes. Mark says Jesus could give no sign to those who did not believe, and they would not have believed (says Jesus in a parable) even if some guy named Lazarus had returned from the dead. John says Jesus performed signs all the time, and as you would expect this led many people to believe in him, especially when he brought Lazarus back from the dead. Though the resurrected disciple who Jesus loved disappears from the narrative after the period John depicts, and even Acts shows no awareness of this important witness.

How can I have morality?? Do I just have to rely on intuition? If the whole world relied on reason alone to make decisions, couldn't we rationalize a LOT of things that we intuit as wrong?

If you want to have morality, you can just do it. By this I mean that any function assigning utility to outcomes in a physically meaningful way appears consistent. But yes, I've come to agree that simple utility functions like maximizing pleasure in the Universe technically fail to capture what I would call moral. For more practical advice, see a lot of this site and perhaps the CFAR link at the top of the page.

Does atheism necessarily lead to nihilism?

This depends. I would normally use the term "nihilism" to mean a uniform utility function, which does not distinguish between actions. This is equivalent to assigning every outcome zero utility. As the previous link shows, plenty of non-uniform utility functions can exist whether Yahweh does or not.

If you mean the lack of a moral authority you can trust absolutely, or that will force you to behave morally, then I would basically say yes. There is no authority anywhere.

What about all the really smart people I know and respect, like my sister and Grandma, who have had their share of doubts but ultimately credit their faith to having experienced extraordinary, miraculous answers to prayer?

Do they seem smarter and more worthy of respect than Gandhi? Perhaps he's not the best example, but putting him next to the many people from non-Christian religions who have made similar claims to religious experience may get the point across. (Aleister Crowley made a detailed study of mystical experience and how to produce it, but you may find him abrasive at best.)

Are rationalists just as guilty of circular reasoning as Christians are?

That also depends on what you mean.