Theories include the idea of religious "memes" as postulated by Dawkins, the evolutionary account of religion, as postulated by Dennett and others, and so on. Other views have sought to argue that religion is merely a crutch, and that God is simply a projection of human needs and longings.
Thus Christianity is viewed as simply an emotional or psychological crutch. Weak people need such beliefs to help them cope, to help them get by in life. Marx for example said that religion was the opiate of the masses. It is like a drug, and keeps them contented with their oppression in this life, knowing a better life awaits them. It is all just 'pie in the sky in the sweet bye and bye'.
Freud also tried to find naturalistic explanations for the origins of religion, He argued that people are afraid of the awesome power and destructive forces of nature, so the god postulate was invented to help tame nature and help people cope. The impersonal forces of nature are personalized and brought under control – we say there are personal spirits in the wind, the storms, etc. These animistic spirit-powers led eventually to monotheism.
Freud could speak about religion as representing "the universal obsessional neurosis of humanity". The infantile craving for parental love and security drives us to posit an illusory divine father figure. Others, like Nietzsche and Feuerbach, said similar things. Religion is just a way humans cope with their psychological needs. Religious beliefs satisfy deep psychological needs.
Feuerbach famously sought to argue that what we refer to as God is simply the aspirations of human projection. God is simply a "dream of the human mind". He also said that "theology is nothing else than anthropology – the knowledge of God nothing else than a knowledge of man".
So how does one respond to such accusations? A number of points can be made. Firstly, it is true that religious beliefs satisfy deep psychological needs. But religious beliefs do far more than this. And even if they do not meet various needs, it still must be determined whether such beliefs are in fact true.
Even if people have psychological needs that are helped by religion, that does not disprove the existence of God. Marx, Freud and Co simply assume the nonexistence of God and then make their case. That does not make for a convincing argument.
And not all religious beliefs are comforting or reassuring. Some can be challenging or repulsive (eg, the doctrine of God's wrath upon sin). The sort of God depicted in Scripture is little like the sort of God people might wish for or long for.
Would people normally desire the sort of God Jesus depicted? He told people that they must deny themselves, take up their cross, and follow him. Paul could speak about putting our sinful desires to death, and glorying in the cross.
Also, it is true that religion can be a crutch for some believers. But nonbelievers also use crutches, whether it be drugs, or alcohol, or fantasy worlds (soap operas, virtual reality games), and so on. We all have props or crutches that we lean on (materialism, hedonism, narcissism, etc). Jesus comes to kick these false props and crutches away. Thus the "psychological motive" argument cuts both ways.
The real question to ask is can your crutch hold you? If you are lame, a crutch is a very handy thing indeed. If you cannot walk, or are crippled or injured, then you very much do need a crutch. The biblical view is that we are all spiritually crippled. We are all damaged by sin. So we need help, and that help lies beyond ourselves. If it weren't for God breaking into our world and helping us out, we would all be in dire straits. Christianity is the cure to our damaged condition.
Moreover, these sorts of arguments commit the genetic fallacy. The genesis or motivation for a belief is a separate issue from the truth of that belief. The truth or falsity of religious beliefs are not determined by their origins. Regardless of why people are religious, we still need to ask whether a certain religious belief is true or not.
As to the various projection theories, they can cut both ways. Maybe the belief in no God is a projection as well. Atheism could also be an illusion, simply the result of human wishing and longing. As Alister McGrath puts it, "if belief in God was a response to a human longing for security, might it not also be argued that atheism was a response to the human desire for autonomy?"
Or as C. Stephen Evans and R. Zachary Manis argue, "If believers sometimes show a deep psychological need to believe in God, nonbeleivers sometimes show an equally deep psychological need to reject any authority over them and to assert themselves as their own lords and masters." If religion can be explained away by sociological or psychological means, then so too can atheism and unbelief.
While many things may not exist which we wish for, it does not follow that something does not exist simply because we strongly wish for it. And as noted, if God is merely the result of wish fulfillment, the God of the Bible is hardly likely to be the sort of God most people would wish for.
Finally, the real question about Christianity is not a psychological one, but an historical one. Did Jesus exist? What did he teach? What did he do? Did he really rise from the dead? These are the real sorts of issues that must be decided upon.
The truth is, people may have wrong or faulty reasons for believing in anything. But the various objections about the psychological and social reasons for religious belief seem to offer little reason to dismiss religion so readily. The misotheists will have to come up with more persuasive arguments than these I am afraid.