Thomas Aquinas was a Dominican priest, philosopher, and scholarly theologian who lived from 1225 to 1274. Among Aquinas’ well-known extant works is his Five Ways, in which he documents five ways or arguments through which one can know that there is a God. They are well worth repeating, so I will do my best to express each of them as follows:

The argument from motion, or the unmoved mover

Some things are in motion
All things that are in motion, were put in motion, or moved by some force or mover
If every object in motion has a mover, then there must have been an original mover
This first mover, who put things in motion, is God

The argument of the first cause

There are in existence, things that are caused
Since nothing can be the cause of itself, all things that are caused have been caused by something else
There can not be an infinite succession of causes going back in time, therefore there must have been an original causer
This original causer is God

The argument from contingent and necessary beings

There are two types of beings in the universe - contingent beings, dependent on another being for its existence, and necessary beings capable of causing the contingent being to exist
Not every being can be contingent since you cannot get a contingent being from nothing
Therefore there must be a necessary being whose existence is not dependent or contingent on any other being
This necessary being is God

The argument from qualitative degrees and perfection


In comparing any two objects, one will find varying degrees of quality or perfection
Degrees of perfection in whatever quality under consideration (strength, intelligence, purity, beauty, etc.) assume the existence of that quality in perfection to serve as the standard by which the quality itself is measured
The perfection of all qualities is God

The argument from intelligent design (also know as the teleological argument)

All things in the natural world behave towards an orderly end or purpose, though they are without intelligence
Behaving towards an orderly end or purpose, however, is a characteristic of intelligence
There must be, therefore, one who guides all things in the natural universe to their purposeful end
This guide is God
Marty Kessler