Aliens are imagined as a living being that don't exist on Earth but on another planet. God's are imagined to exist in another realm with powers to destroy and create life and who have differing personality's or roles depending on the religion. Those personality's or roles do not reflect in every day life so we dismiss the existence of them, after all how can the Christian god be omnipotent yet is under able to kill/stop Satan and if the commandments say "We shall not kill" why are we able to ask for forgiveness to enter heaven? Last I checked every action has a consequence yet gods seem to tell us, it doesn't matter what you do, you will enter heaven anyway if you bow down to me/ask forgiveness. So yea one is a load of bollocks and one is actually a possibility.