Thomas Aquinas’ ‘Five Ways’
Aquinas believed it was possible to discern truths about God based on reason (human rationality) and revelation (divinely revealed truths not available to reason). Aquinas lived at a time when Aristotle’s teachings were popular and so used this as basis for his theological writings.
This is a foundational piece of philosophy for much of the Roman Catholic and Protestant Church today in that it details the fundamental premise that existence implies origin.
Later philosophers have disputed this — thinking that it is just as easy to postulate an eternal universe as an eternal God. However, such can be easily addressed through the realization that our assumptions about the cosmology and it’s functionality may, themselves, be limited to that universe and, as such, incomplete. In such, an eternal origin (AKA. God) is only slightly more plausible — but enough to make the examination of nature for signs of created (as opposed to random) origin a worthy pursuit.
As such, this is a critical piece of philosophy that everyone in the church should understand for it forms the philosophical fork in the road between Christianity and atheism.