"The entire domain of conventional religion is (commonly) the domain of immaturity — or of childishness and adolescence, rather than real human maturity."
When people believe in "God", what they are actually believing is that everything that is outside of themselves is ultimately epitomized in some Person, Force, or Being that is not merely making and controlling everything, but is in charge and is going to protect them — and, especially, that this "Other" Person will protect them and even help them to get a lot of things they want, if they will enter into a special kind of relationship with that One.
That relationship is very similar to the one that you were called to enter into with your parents: "Be good — and we will love you, and protect you, and give you things that you want."
Thus, popular (or conventional) religion is largely a cultural domain of social morality. People are asked to behave in one or another fashion that one would call "good" in order to maintain a good association with the parent-like "God", so that they will be loved and protected by that One and given the things they want (while they are alive, and after death).
Conventional religion is largely an enterprise of childhood — of the dependent, childish state.
When people become adults, however, they have more hard facts to deal with in life. They feel much less protected than they did as children in the household of their parents. So they begin to question and to doubt the existence of this Parental Deity.
Such individuals may continue to be conventionally religious in some sense, willing to play the game of social morality and good behavior — but they carry on a rather adolescent relationship of dependence-independence, or embrace and withdrawal, relative to this "God-Person".
Atheism is the ultimate form of denial of the Parental "God". Atheism is not founded on real experience of the ultimate facts of the universe. Rather, it is a kind of adolescent development of the human species.
What characterizes the doctrine (or dogma) of atheism is not a discovery that there is no "God", but a refusal to acknowledge every kind of parent (or parent-like authority), including (therefore) the Parental "God" of childish religion.
If conventional religion amounts to an actual experience (rather than just a kind of conventionally acquired state of mind), it could basically be defined as a very primitive sense that invades all of your life, but that relates to you most specifically in your solitariness, your private individuality.
It is the sense that, when you are alone — and you are, in the sense that you have a private destiny, always alone — there is always "Somebody Else", the "Great Parent", always there.
That One, it is presumed, sees everything you do, and represents a "Parental Will" relative to what you do. That One wants you to do certain things, wants you not to do other things, and will presumably reward you if you do the things that It wants you to do and will punish you in various ways if you do not do those things.
