The Cosmological argument originated from Plato and Aristotle however it was mainly later developed by St. Thomas Aquinas. Their arguments both began with the idea that motion needs a prior agency. Plato then identified the first cause of the chain of events as the need for an unmoved mover which started off the chain.
Aquinas' main argument is well known as 'Aquinas' third way'; the argument from contingency and necessity. The first of Aquinas' ways was 'from motion,' this follows the idea that all objects move and a change of quality is movement. Nothing can move itself, which then leads to the idea of a chain of movement but the chain cannot be infinate, therfor there must be an unmoved mover to begin the chain. This first mover is God. The second of Aquinas' ways was 'from efficiant causes,' this follows the idea that all things are caused by something else because they cant cause themselves or they would exist before themselves. However this would mean that there cant be an infinite chain of causes, meaning there must be a 1st cause that caused all causes, then this 1st cause is God. The third of Aquinas' ways is 'from contingency and