I've never had any doubt that all religions are man made. Their obvious contradictions, plagiarisms, and child-like observations of the natural world around them convince me beyond any reasonable doubt to their falsity. With Christianity, we are told that we are sinners from the moment of conception, that we are born sick and commanded to be perfect, that we don't deserve the life that we didn't even ask for, and that we all deserve the Christian hell by default. Now to me the masochism here is obviously apparent. This guilt ridden wallowing on one's knees in shame and terror of one's natural state, is tantamount to the slave who enjoys being urinated on and whipped.
Not that there's anything wrong with wanting to be urinated on and whipped, but when your whole life outlook concerning human nature, sexuality, desire, and existence, is from the point of view of the self-loathing masochist via the Christian mindset, and you are promoting this view towards others, that is when it becomes a problem. As atheists we do not believe in an inherent state of "sin", and we don't need to feel guilty of natural desires as long as they are not harmful to others. There is no need to seek approval from some invisible authority that we can't see, hear, touch, feel or measure in any way to justify our very existence.
The freedom involved with being an atheist is something that makes some theists anger. While they shackle themselves psychologically in manacles submitting to an invisible master, they see us laughing and enjoying our lives free from a harmful masochistic complex in our outlook on life and nature. They see us indulging in activities they consider sin, and sometimes boil with rage as to why we are not also swimming in guilt. Many Christians also want to export their Christian guilt onto us so that we too become self loathing and submit to their invisible god. The atheistic outlook does not necessary sanction a totally hedonistic indulgence for one to engage unmitigated in every desire because every rational person understands that we have to live in a society with rules. The problem is that the rules of Christianity are not based on reason, they're based on the ignorance and superstition of masochists.