If I were the Devil, I would masquerade as the creator of the universe, pretending that I entered the universe as a man and was sacrificed for the way I myself created human beings. I would custom-tailor a cluster of memes to capture their minds, convincing them that they would suffer forever unless they worshiped me, that there is no morality without me, and that they owe everything to me.
I would laugh at them as they looked up at the stars and asked me for help -- I would continue to punish them with disease, natural disasters, and a virtually unlimited range of physical and mental harm as they praised me. I would convince them that this suffering was THEIR fault! Hahah. And all I'd have to do to effect this outcome was present myself to them and tweak their brains so that a warm, fuzzy feeling resulted.
The best part would be the finale; I'd imbue them all with certainty that they were going to a Disneyland in the sky upon physical death, giving them false confidence and allowing them to think they were destined for something greater than the rest of humanity, then upon death inform them that they'd been worshiping the source of suffering all along; that a genuine loving entity wouldn't have done any of this. Then I'd say 'toodles' and begin anew with the next generation.
In other words, if I were the Devil I'd continue doing what he is doing today.
Well, regardless of whether or not this God stuff is nonsense you're getting exactly what you want.
If God is completely bogus, you've lived your life as you wanted to without any religious nonsense. If God is real then you still get to live your life without the nonsense and when you die you get to spend eternity without God then too. You can enter eternity, seperate yourself from God and all of God's attributes and lock the door between you and God from the inside.
For me, if God is completely bogus I'll have spent my life dedicated to something that isn't real and willingly wasted time worshipping something that was never there to begin with; although, when I die I won't know the difference as I'll simply cease to be and will have no realization, not a single instant, that it wasn't real. If God is real then I've lived my life for him and then get to spend eternity with him.
That said, why would I be willing to take such a gamble with my life with "no proof whatsoever"? Why would I willingly surrender my life for God? There has to be something else there that nonbelievers don't understand. I mean, I know you summarized the Holy Spirit with "tweak their brains so that a warm, fuzzy feeling resulted", but maybe there's still something more to it than that despite best efforts to reduce it all away? Maybe I'm just crazy, maybe I'm delusional, maybe I'm brainwashed, maybe I'm on drugs, maybe I'm uneducated, maybe I'm a moron or maybe.....just maybe.....I've experienced the risen Christ and it was more than a brain-tweaked, warm-fuzzy feeling beyond your best efforts to be reasoned away and reduced to nothing.