The classic problem of evil/pain is a big one for me. I.e., how does one explain the existence of evil when God should want to eradicate it, He is capable of doing so, and He has the know-how to do so.
Setting aside babies dying in earthquakes (natural evil, which I think is hardest for believers in the Christian God to justify), I’d like to address personal evil.
Almost every discussion I have with a Christian inevitably comes back to free will. A poster on this GD board recently put it thusly: “The only way to be sure that nothing undesirable will happen is to refuse the freedom of choice.”
But, that argument has never worked for me. I don’t think free will explains the existence of undesireable things.
If God has free will, and yet, over an eternity of choices, He never chooses to do evil, not even once, then certainly free will, by itself, is not enough to justify the existence of suffering. There must be something different about us that makes us susceptible to evil, something that He put in us or left out of us, presumably on purpose, something other than free will, which leads to evil/pain.