I agree with both niminy and sieglinde's points about how our lives are, in many ways, mostly predicated on non-rational (but not irrational) things.
I am also thinking about the "experience of God" and how it is experienced. It might be contended that it's "just chemicals in the brain", but I don't think there is anything "just" about that. We are creatures of flesh and chemical signals, so it seems obvious to me that of course, naturally, our experience of God is going to be mediated through that, and not through something completely "other" that we can't identify yet. We experience life through our bodies, so it seems obvious that we would experience God through our bodies (how else would we experience God?). This doesn't negate the experience. We also experience love and beauty and creativity through the natural human processes of our bodies, brains and minds but non-theists don't dismiss these experiences because they have arisen out of our physiology.
In the end, the things that seem to be most important to people in life - generally speaking - are not the rational things that we know and can prove but the non-rational: the emotional lives of people - those whom they love - humans and pets -, the music they choose to listen to or books they are stimulated or moved by, the places they cherish, the beauty they find in things, their pleasure in developing a skill (whether in music, or art, or sport or craftwork etc). Even if it were "proved" that preferring Bach to Vivaldi conferred some distinct evolutionary advantage, how does it explain the creativity, the very idea of "liking" something having meaning in our lives? If emotions and "experiences" are capable of being dismissed because they are "just" chemical processes in our brain and nothing more, then how does any non-theist actually 'believe' in them at all? Surely they are meaningless processes, like any other in the universe, and not to be trusted?
Is it a kind of self-deception or trick? - that a person might rationally assert that emotions are all just a set of chemical and electrical signals, designed to confer some theoretical advantage, whilst simultaneously subscribing every day to what those emotions tell us. If love is just chemicals, does the phrase "I love my wife" have any meaning beyond the chemicals?
Non-theists presumably consider there is no meaning in anything in the universe? In fact, that "meaning" doesn't exist at all? There is no purpose to anything? What I find interesting is that this would mean that all of human endeavour, relationships, creativity, is also ultimately meaningless and a basic trick of human physiology; it would mean that humans basically pretend all of the above "non-rational" qualities actually "mean something" so that they can deliver our genes effectively to the next generation.
If there is no Meaning in the universe and nothing beyond the material, then how does a person find meaning and why should they?
eg. a non-theist might posit, "I know that scientifically the "feelings of love" I have for my wife are actually just random firings of chemicals and electrical signals and "mean" nothing ultimately, physically, other than a slight evolutionary advantage for pair-bonding and longer survival, but I'm going to live as if "love for my wife" actually means what it says on the tin, that the emotions are not just a vehicle for survival but important and meaningful in their own right, and will involve all kinds of non-rational things like loving visiting art galleries together or watching a film together, or helping her with her craft project or visiting my old school friend together, or writing a poem for her". In the end, how does a person live thinking that "love" and every loving transaction, creativity and every creative endeavour, is nothing more than merely a tool for survival?