To say religion is a man made concept seems critical, but it is 100% belief. There is the possibility that all religions have man-made concepts, that one religion is completely inspired, or that all religions are completely inspired by God to fill the people's need. To be critical, you need not deny the possibility of anything being true, but only to be aware of the other possibilities.
they are man-made concepts, everything about religion has to do with man, a being of infinite power would not allow people to enslave his children or let them murder each other or make laws allowing killing if you break a law
what you are arguing isn't really critical thinking, its more like you are arguing that because we don't know all possiblities that the ones we do know are irrelevent and we shouldn't trust them
the thing is all religions are made from our standpoint not gods, man worships gods, so yes not only are religions man-made but the concept of worshiping a higher being is purely a man-made concept, didn't your god create us to choose? well we don't have to believe in anything or anyone.. its a choice
That does not and should not detract from or change the possible validity of the religion.
but theres nothing that shows any religion is right, all the books, beliefs and everything else doesn't show them right.