Religion does not exist because of evidence/proof for it, or evidence against it, or a lack of evidence either way. Anyone arguing for this is lost. The reason religions or spiritual beliefs exist is simple, and it has nothing to do with proving if God exists or not:

Nevertheless, another reading can be made of Marx. The often cited phrase--"religion is the opium of the people"--is truncated. What follows this remark lets it be understood that human beings need opium, because they are metaphysical animals who cannot avoid asking themselves questions about the meaning of life. They give what answers they can, either adopting those offered by religion or inventing new ones, or else they avoid worrying about them.

As for why the major monotheistic religions, mainly the Abrahamic ones, operate in the way they do, Marx helps explain it. Man created religion, religion did not create man. So what religion states is above in heaven is used to legitimise the current order of what is taking place on earth. Thus from there, critique of religion becomes critique of law, politics, social organisation and society itself, as Marx states. I will give an example below if how religion has been used to legitimise patriarchy, and how critique of that eventually becomes critique of religious fundamentalism.

Is monotheism really a wonderful advance in the history of thought, a qualitative progress? There are plenty of cunning minds (but when you say cunning, you could as well say ill-intentioned or malign, inspired by the Devil) who draw a parallel between this unique God (who is represented in the popular imagination, if not in the purified vision of the learned, as an old man with a white beard, a symbol of wisdom and authority) and the patriarch of the patriarchal system, the autocrat of the power systems. In this imagery, which adequately reflects what is actually experienced, it is obvious that the wise old male is closer to God than a woman or a youth. This is a projection into heaven that legitimizes the patriarchal order and autocracy which prevails on earth. In addition, the elimination of female deities, always important in non-monotheist religions, only accentuates patriarchal domination. Those cunning minds will add that this only and all powerful God deprives them, poor bastfards, of all power.

• All quotes are from Eurocentrism: Modernity, Religion, and Democracy. A Critique of Eurocentrism and Culturalism by Samir Amin.