The most simple way to begin is that God is the truth.
Why does it truth matter? Because false conclusions lead to false judgement and throws off the balance of what is true. If I think there is a stair below my foot when there is really a three-foot drop, I am going to fall and get hurt. If I think that the world is flat when it is really round, I am going to be too filled with fear to sail very far from my home and miss out on a diversity of culture. So if I think that God does not exist when in fact He does, what will that do?
First of all, it leads to the conclusion that there is no meaning to life. I was at a sleepover once and a friend asked “What would you do if you knew you had one day to live?”. I could not think of anything to do except go to confession, go to church, and hope. My non-religious friend said she would get drunk, and I couldn’t help but think…why would you do anything? If you “knew” that within 24 hours you would simply not exist, then why does it matter if you get drunk or get laid or discover a new planet or stay at home and sleep? Why does it matter that you were even born? If I were an atheist and I knew I had one day to live, I’d kill myself and get it over with.
Second of all, it leads to a complete lack of a moral code. Every civilization of the world is run on some sort of a moral code, what you should do and what you should not. However, you may have noticed these codes breaking down. The brand new secular generation questions “Why not? Why not do drugs, why not have sex all over the place, why not kill people, why not?”. And they are absolutely right. Without religion, without God, there is no such thing as an objective moral code, or any moral code. This generation says “As long as you don’t violate other people, you can do whatever. Just don’t hurt other people”. What if the next generation doesn’t even say that? Most will deny it, but without a singular ruling force (God) structure breaks down and nobody can say what is actually right or wrong anymore.
I’ll think of more later and edit this. But that’s my two cents so far.