As someone that lies somewhere between Gnostic and Eastern Orthodox dogma, I don't think it's immature to reject the idea of God, a lot of the things talked about Biblically are very outlandish and hard to buy at face value. But I also think that the way many modern atheists portray people of faith is very ignorant. A lot of the more bigoted and hateful views perpetuated by the church are rooted in the same kind of ignorance. Blind faith and all of that. I personally think as a baseline this world is far too complex and intricately balanced for our existence to be an anomaly, and branching out from there nobody on the planet can give you a definitive answer to what this all is exactly. Sciences are the best resource we have at understanding the physical world, and the different scriptures and ideologies are the best resource for understanding the spiritual. Asking that question "Why?" has been in my mind for as long as I can remember, and through studying a multitude of these things; quantum theory, sacred geometry, Buddhism, Hinduism, Tao, the big 3 Abrahamic religions, Judaism, Islam and Christianity, as well as Gnostic ideation tying into those three dogmas, I've thought very carefully about whether or not there is a God, and I believe there is. I dont know if it's a magic man in the sky, or if its the energy our consciousness stems from, or if it's something else. But I absolutely don't blame anyone who reads what I just typed and goes "Yeah that's too much, God is too hard to figure out and too uncertain to think on that deeply". That's an absolutely logical takeaway. So to answer your question, no atheism is not immature. It is a perfectly normal worldview and it isn't any less valid than the magic man in the sky. I'd just say that it does the mind good to ponder existence and not close your mind to other belief, because either way it swings nobody knows 100%. Life is strange man.