During his debate with Arif Ahmed and Andrew Copson at the Cambridge Union Society, Peter S. Williams gave a lucid illustration for the argument for the existence of God based on the contingency of material reality.[1]

Imagine if I asked you to loan me a book.  You say you don’t have it, but you’ll ask your friend to loan you his, and in turn you’ll loan it to me.  When you ask your friend for the book, he says he does not have it, but he’ll ask his friend to borrow his copy, and in turn he’ll loan it to you, who will loan it to me.  If this process continues ad infinitum, I will never receive the book.  If I do receive the book it is because the process of requesting to borrow the book is not infinite, but temporally finite.  Somewhere down the chain of requests to borrow the book, someone actually had the book without having to borrow it from someone else.  

Similarly, nothing we observe in the universe has necessary existence (everything is contingent). Everything has to derive its existence from something that existed prior to it. Clearly this process cannot continue on for an infinite time in the past for the same reason the process of borrowing a book cannot continue for an infinite time in the past. If nothing in the past has existence in itself (necessary existence), then no contingent beings would exist in the present.  Since contingent beings do exist in the present, there must be a necessary being from whom they derive their existence—a being that does not derive its existence from anything else, but has existence in itself.  Williams went on to quote philosopher Richard Pirtle who similarly argued, “Consider any contingent reality. … If the process of everything getting its existence from something else went on to infinity, then the thing in question would never have existence.  And if the thing has existence, then the process can’t have gone on to infinity.  There was something that had existence without having to receive it from something else.”  

If God did not exist, nothing else could exist.  Since contingent beings do exist, God exists.  

[1]Starting at 12:44.