The first cause argument is an argument for the existence of God associated with St Thomas Aquinas (1225-1274).
Aquinas was a monk who used reason and logic to point to the existence of God.
As human beings we are used to seeing cause and effect in our everyday lives, so this argument is easy to relate to. For example, if you push someone (cause) they fall over (effect).
Aquinas argued that our world works in the same way. Someone or something must have caused the world to exist. The cause is God, the effect is the world.
Aquinas argued that this first cause must have no beginning - that is, nothing caused it to exist because the first cause is eternal.
He argued that this first cause is God. God is eternal (has no beginning, was never started) and God caused the world and everything else to exist.
Not everyone accepts the first cause theory.
The first major problem is that we have no answer to the question ‘Who caused (created) God?’. If everything requires a cause (something to start it) surely this has to apply to God as well.
If some people can believe that God is eternal and requires no cause, then surely you could argue that the universe is eternal, and so doesn’t require God for it to exist.
If you can apply the principle that one thing is eternal (God) then surely that can be applied to other things (the world).
For religious people, the first cause makes God into some kind of distant being who simply made the world. This does not fit with the God of most religions who seems to be involved in caring for the world, not just the being who started it.