To understand where Christians are coming from, this most important and first thing to wrap your head around is that religion, in general, and Christianity, in particular, is not based on reason. First, let me explain what I mean by rational and irrational. Irrational is not an insult, nor do I mean that when people act irrationally, that that means what they doing is stupid or insane. Unfortunately, there is not a word, that distinguishes the spiritual rational, from the physical rational, in philosophy.
Just like the word Modernism has nothing to do with Contemporary, as used in Philosophy, the word Rationalism refers to a certain type of philosophy and is not a value judgement. We could substitute a word like utility or practical, to distinguish between spiritual and physical reasoning, but we run into the same problems, because spiritual things can be very practical (ethics and even sometimes the supernatural) and have clear utility (they help keep our society together and provide hope and purpose in life).
Whenever someone uses the word spiritual, many people think that means supernatural, but ethics are a big part of spiritual reality and most sins are only possible in a physical world. I think a better way of breaking down the difference between our different modes of existence are the different aspect of our self awareness, including the spirit (the mind, the heart, the will/soul), and the body. I would say that Christianity sees the will, as being the primary actor, in our world, not the body or mind, as Science teaches. Psychology deals with the heart but, while seeing the connection to the mind and body, fails to factor in the state of the will.
So Christianity is not about why or what but about Who. If God did not exist, then praying to Him would do nothing and nothing (including us) would not even exist, in the first place. While there are many rational reasons, to believe in God and to explain it to others, the concept of faith is not defined by belief in what we can understand. God doesn't ask us to believe He merely exists. God calls us to choose to obey Him, by repenting of our sins, believing in His worldview, and asking Him to interfere, in all aspects of our lives, so that we continue to choose His will, instead of ours.
We do good for others and follow His rules, not because we have to do so, to be saved, but that, if our faith is real, we really want to do the right things, because God has changed the way we think and our priorities in life. We help others, because we are thankful that God helped and helps us. Many non-Christians do not understand why someone would feel they need to change themselves for God to accept them, but they do not hesitate to totally transform their beliefs and their lifestyle, just to gain one weak human friendship.
God takes us where we are and we change, because we realize, that God knows what is best for us (as He mades us and lived as us, as Jesus Christ) and following Him will improve our lives. God is not interested in: what you do for a living, what kind of personality you have, what your artistic views are, what you have done in the past, where you live, or other less significant things. God cares about who you are now and He is just there to make you a better person and give you hope and meaning, despite whatever circumstances you have found yourself in or what decisions you have made, good or bad.
So whether or not religion is rational or not rational, more importantly it makes your life better and Christianity does so be God working directly in your life. Isn't it amazing that God cares so much for us, after all the terrible things we have done and how weak, stupid, and unimportant we are, in the grand scheme of things. Think about how big the universe is and think how sparsely populated it is: God went to great lengths to make our lives possible. If the conditions were slightly different, in physical laws, the universe would not hold together and fire and flight are possible on earth only due to a very specific range of properties.
Christianity is often thought, to be in the dark ages, in some of the beliefs we follow, that are written in the Bible, compared to what non-believers believe today. If you want to see something truly ridiculous, just look a couple hundred years back in Science and compare to what Scientists believe today. I remember, when I was kid, black holes were thought to be impossible and now they are thought to be micro ones everywhere and the center of each galaxy is a great black hole. Stephen Hawkings even tried to prove the whole universe is one big black hole. Now there is a direct correlation between the necessary chemical components needed for our form of life and black holes.
Who knows what Science will say about the world in the next 200 years, but the Bible will still say the same thing it said 2,000 years before us and it will still be the most widespread religion in the world. Christianity is already outpacing Islam 2:1 today and growing more rapidly, in some of the fastest growing populations, than Islam (and relevant to many more parts of thew world than Islam ever will be). The Islamic world has no faith in the dedication of fellow Muslims or else it would not be a death penalty, to spread the Gospel or convert to Christianity, in most of the Islamic world, even today.
Most of the modern Islamic populated world was Christian (including the entire Middle East), for 500 years before Islam left Saudi Arabia. This situation continued up until 2/3 of their (Assyrian/Syriac) population was massacred between World War I and II. This was the culmination of ethnic cleansing and genocide of Christian areas by Muslims, for hundreds of years, by radical Islamic fundamentalists (much as Jewish people were persecuted in Medieval Europe). Most of the world (outside the Middle East and Europe) only heard the Gospel a century ago, while Islam has been in many of these countries 500-1,000 years before.