Most religions require a blind acceptance of their particular belief system. There is an implicit understanding that you cannot question these beliefs. This resistance to questioning shows the fundamental instability of religion. If you were truly convinced that you were right you would strongly encourage questioning. When questioning is disallowed it implies that there are embarrassing questions that cannot be answered.
Most religions, particularly the most popular ones, contain a great core of wisdom. The early creators and maintainers of the worlds religions had a huge problem though. The common people greatly needed to be taught this wisdom. The problem was that the common people were not that interested. Often they were just trying to survive. They did not have time to think about living well and being good to each other. The stewards of the religions therefore needed a hook to entice common people to look at their religion. So they created captivating stories and myths sometimes interspersed with remnants of truth and wise teachings.
Religions are also subject to gradual changes over time. Sometimes these changes are due to translation errors or even deliberate distortion. If you examine ancient religious texts, what they say is quite different from what is now being disseminated as gospel. Religious splinter groups also get created when a charismatic leader appears and distorts the original religion in order to satisfy their own needs of power and control over others. Such leaders often have personality characteristics of psychopaths, narcissists and other malignant personality disorders.
One big problem with religion is the common theme of God as creator. If there was a god that created the Universe, this implies that something must have created god and so on. This is known as the creator paradox. How can such an infinite regression exist? It's much simpler to equate God with the Universe and nothing more. Maybe something created the Universe but how would we ever know? How can we believe in something that has no basis in fact?