Most atheists might say "there is no evidence for God, people who believe in him are dumb", but this is not my issue, people are born more or less intelligent than others anyway. My issue is that worshipping God is not just stupid, but IMMORAL.
Think about all the children who die in terrible agony around the world, maybe because of an illness or a natural disaster, even at the very moment you're reading this thread. Their parents are praying for them to be saved, but those children are going to die regardless. God won't save them. Why? There are only 3 possible explainations for this: God is impotent, God doesn't care, or it's all part of God's plan.
If God is impotent, it means he's not a god by definition, and therefore worshipping him is immoral.
If God doesn't care about people's suffering while having the means to easily help them, it means he is evil, and therefore worshipping him is also immoral.