"Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people. The abolition of religion as the illusory happiness of the people is the demand for their real happiness. To call on them to give up their illusions about their condition is to call on them to give up a condition that requires illusions. The criticism of religion is, therefore, in embryo, the criticism of that vale of tears of which religion is the halo. Criticism has plucked the imaginary flowers on the chain not in order that man shall continue to bear that chain without fantasy or consolation, but so that he shall throw off the chain and pluck the living flower."
~ Karl Marx
It serves a multitude of benefits for the majority of the population. It's one of the best evolutionary advantages nature and God gave us in order to maximize our lives.
I do not even understand by what you mean by saying "grow up". First off, if you had any idea or understanding of history you would know why people have shared and passed religions on for thousands of years.
And then of course, you have the amazing enlightenment thinkers who started deism and then branched to complete athiesm.
Having a religion does not mean you are immature. It means you hold on to certain values you were raised to or have discovered.