The argument from contingency draws on the distinction between things that exist necessarily and things that exist contingently. Something is “necessary” if it could not possibly have failed to exist. Something is “contingent” if it is not necessary, i.e. if it could have failed to exist. Most things seem to exist contingently.
All of the human artefacts around us might not have existed; for each one of them, whoever made it might have decided not to do so. Their existence, therefore, is contingent.
The argument from contingency rests on the claim that the universe, as a whole, is contingent. It is not only the universe, the argument suggests, that each of the things around is us contingent. It might have been the case that nothing existed at all. The state of affairs in which nothing existed at all is a logically possible state of affairs, even though it is not the actual state of affairs.
If the universe might not have existed, then why does it exist?
If the universe might not have existed, then why does it exist? The existence of things that are necessary does not require explanation; their non-existence is impossible. The existence of anything contingent, however, does require explanation. They might not have existed, and so there must be some reason that they do so.
(1) Everything that exists contingently has a reason for its existence.
(2) The universe exists contingently.
(3) The universe has a reason for its existence.
(4) If the universe has a reason for its existence then that reason is God. Therefore:
(5) God exists.
If you have read On Guard, then this is the level up and may feel like quite a leap in terms of depth. Here you will get a fuller, more in-depth case and defence of the arguments you learnt about in On Guard and this applies for the Contingency argument chapters with a larger analysis of opposing theories. This book also comes with a study guide
A level up from Reasonable Faith book and is very much aimed at scholars and serious students of natural theology. If you want to get into the serious arguments defending Christianity, this is the place to look, but be prepared for a complex read, even if you’re quite comfortable on the contingency argument material so far.