I was asked this question by a friend, when I was in College. My first year in college, I had been back from my Mission for... maybe 6 months. I think it came up in part because he had been proselyted by a group of religious atheists - I have always found that aspect of Atheism a bit humorous. They eschew religion while being a religion themselves - In the sense that a religion is an organized collection of beliefs, or world views relating to humanity, and providing some sort of structure. I am not referring to atheists in general, but rather that group of zealous atheists who preach with conviction that science has proven the non-existence of God. Science can neither prove, nor disprove the existence of God at the present time, nor is it likely to be able to in the foreseeable future. You may be able to make a case regarding certain qualities of an intelligent being, but not dis/prove one.
But back to the question. "What if it isn't true?" At the time I was grossly incapable of answering the question. Fresh from a mission, I was still positively brimming with missionary-ness. I could not even comprehend "It" not being true. I have matured somewhat in the ensuing years (probably not much), and thought I would take a stab at answering that question now.
So, what if it isn't true? What if there is no God? What if all those we have been following over the years, believing they were "men of God" were all part of an elaborate conspiracy, or more likely (applying Hanlon's Razor), were men with an incredible abundance of imagination, and an incredible lack of ability to distinguish between fact and fancy? Well, then I have part of an enormous fraud, which has been perpetuated through millenia. I have been giving up 10% percent of my income, untold hours of my time - I've been following a bunch of rules and guideline's - all for no reason.
But what does that really mean for me? Ignoring the human frailties which are part of any organization run by humans, whether founded by a divine being or no, what are the actually fundamental implications of my devotion to this fraud? What are the actually tenets and objectives which the Church itself - not individuals within the church - has attempted to instill in me?
At the core, it is all about aspiring to something greater. The most fundamental, core doctrines are love, and personal accountability. Seriously, take some time to pick it all apart, lay it out, and look at the big picture, recognize the influence of stupid, which is part of all human endeavor and distill the core meaning. It is all about taking responsibility for my actions, accepting my faults, and seeking to overcome them, it is about not blaming others but recognizing that I am responsible for me. And then, in what seems to be a contradiction of the former, it is about recognizing the influence which I have on others. About understanding that what I do can influence others, can shape others. And because of this core belief that we are the offspring of a deity, Those "others" are literally my brothers and sisters. All children of a Loving God and all therefore of infinite worth to him. If that is the case, then why would I not do all in my power to help them to achieve their full potential?
Putting that in simple, temporal terms, If I have a dear friend, who I know is a recovering alcoholic, am I going to pressure him to go to the Bar with me? NO! To do so, to put my friend at risk of Jail, of Death... it would be unconscionable! So in that same vein, if all around me are my brothers and sisters, it makes sense that I should do what I am able to help them to be happy, healthy... the best they can be.
And that is the core of what I have belonged to. The gospel of Jesus Christ fundamentally is about improving myself, being responsible for myself, avoiding things which could lead to addictions, and embracing things which improve my knowledge, health, happiness and overall well-being, and helping others as best I can, or at the very least, not being a roadblock to their improvement.
So, I try to avoid things which are harmful for my physical body, I try to eat good food, get sufficient sleep (I said try), I try to treat other with respect, to keep my anger and selfishness and pride in check. I give money and time to my church, to build church building where people can gather together to fast and pray together, to share messages of hope and faith with one another, to inspire each other and "bear one anothers burdens". I give money to my church to help build schools, to help provide resources to those who have fallen on hard times; see to their immediate needs and help them get back on their feet.
I have invested considerable time in an organization which has encouraged me to follow the example of one who spent his life seeking to help, to heal, to lift, and to love.
What if it isn't true? What I have gained far exceeds what I have lost.
What if it is true?