St Thomas Aquinas (1225 – 1274) developed the most popular argument as a 'way' (not proof) of showing that there must be a God.
Aquinas argued that everything in the cosmos has a cause. If you track things back through a series of causes, there must have been a 'first cause'. He said that this 'first cause' is God, whom he described as a 'necessary being', eternal and transcendent, existing outside of our space and time but able to act within it, needing no explanation and having no cause.
Strengths of the argument
Scientific discoveries, eg the Big Bang theory, can be seen to support the first cause argument. If God caused the 'Big Bang', then God is the 'first cause' that brought the cosmos (universe) into existence.
It confirms to the theist that there is purpose to the cosmos and a place for God as its 'creator'.
Weaknesses of the argument
If the argument is based on the idea that everything has a cause, then this leaves open the question 'Who or what caused God?' To reply that God needs no explanation is not enough to prove God's existence.
The Big Bang was not necessarily caused by God – it could have happened by chance.
The argument is presented for believers and makes sense to them, but it is not convincing for the atheist or the agnostic.