We believe - formally at any rate - in a God who's omnipotent, onmiscient, and all the rest. He's the biggest the strongest, the king of the ubercastle. Yet we hear that it's possible for the human will to trump God's. God wills our salvation, yet we can resist it, and go marching off obstinately into hell. It's never made a lot of sense to me; I could easily have been a Calvinist - which at least makes logical sense - except I can't stomach a God who acts like a bigger and better version of Adolf Eichmann, sending people off to the eternal gas chambers.
Once you have the idea that God 'can't' do X, it's not a big step to say he 'can't' do Y either, and before long we have a God who can't do very much at all. Seriously, I've listened to preachers - fortunately not Methodist ones - who seemed to think that God couldn't do anything for us unless he was assisted by our faith. No salvation without our faith, no healing without our faith, and so on. It gets very Deuteronomic; if things aren't working out for you, it's your fault for not having faith.
That's a blatant example, but we meet a subtler one the whole time. All possibility of salvation ends if we die in our sins. Death is bigger than God. That's not what Paul thought, though, is it? I detest proof texts; you can prove anything by quoting half a dozen words out of context, buttressed by 'The Bible says'. Trouble is, what it says in one place, it often unsays in another. You want to promote infanticide? You'll find a nice quote in Psalm 137:9. But Paul undoubtedly does say 'The last enemy to be destroyed is death.' (1 Corinthians 15:25-26), so at least he thinks God is the stronger of the two. But if death can die, what price mercy after death? An omnipotent God surely isn't going to be beaten by the mere ending of bodily life.
I'm not going to pretend I know what happens after death; only God and the dead can answer that one. But all Christians agree that there's hope (one of the very few things they do all agree on!), and we may be mistaken in putting limits on that hope. I don't know whether everyone ends up being 'saved' or not, but if they are, that seems to me to make a lot more sense that the God who wants to save them all, but simply can't manage it.