There are many compelling reasons to believe that an all-powerful, all-good God does not exist:
There is too much suffering. If God exists, he permits plague, war, genocide, natural disasters, mental illness, and much more, all of which he has the power to prevent. Some believers say God must have reasons for allowing such things, even if we don't know what they are. There is, however, no reason to believe that such reasons exist, and every reason to expect that a world created by an all-powerful and all-loving God would not need to include holocausts and black deaths. Every time another such event takes place, we must believe that God has yet another unknown and inexplicable reason for permitting it. The more such unknown reasons we must accept, the more evidence we gain that such a God does not in fact exist.
Religions are too parochial. Many religions believe that God chose reveal his teachings at a specific time to a specific group of people, thereby leaving large swaths of humanity largely or completely ignorant of him. This is not what we would expect from an all-loving, all-powerful God who wished to draw all humanity to him, but it is what we would expect if each religion is an outgrowth of a particular human culture.
There is too much religious confusion. Believers of many different faiths report similar experiences of God speaking to them, guiding them, and comforting them. If God existed and wanted humans to follow his true path, we would not expect to see so many people experiencing God in such different and conflicting ways. We would expect God would make himself clearer to mankind, rather than providing so many conflicting religious experiences and manifestations. Such a degree of religious confusion is far more understandable if religious feelings and experiences are instead solely the product of human psychology and society.
God is a poor explanation for anything. God cannot explain why the universe exists, but merely pushes back the question, for we can then ask why God himself exists. Likewise, God's existence cannot explain human consciousness, for any talk of immaterial souls or spirits merely applies a new label without actually saying anything about how or why consciousness arises. The ability of God to provide answers to such questions, therefore, is illusory, leaving us without any strong reason to believe in such a God. —James Fodor