The Cosmological Proof for God's Existence

Things exist - this is a fact known by our senses.

A thing cannot cause its own existence because it would have had to exist before its existence in order to create itself, but that is a contradiction because no created thing can exist before it is created. So, a thing either has to be contingent (ie receiving existence from something else) or necessary, (which means that it must be existence).

THERE ARE ONLY TWO WAYS TO EXIST:

CONTINGENT receiving existence

Or

NECESSARY being existence itself

A contingent being cannot be the ultimate cause of the existence of something else since it has only received existence. The only explanation for contingent beings is a necessary being. The necessary being is given the name God.

The Law of Causality states that: “Anything which begins to exist must have been brought into existence by something distinct from itself.”

If we consider all created things (let's call them creatures), something caused the existence of every single one of them. Let's consider one of them. It could be a giraffe, a planet, a human being or any other creature you care to think of. Let's consider a giraffe.

What caused the giraffe?
Other giraffes. Its parents, to be exact.
What caused the giraffe's parents?
Their parents.
What caused the giraffe's parents' parents?
Their parents.

And so on, and so on until we go mad or end up realising that it does not make sense to say that created things have been created perpetually by other created things with no beginning to it all. We cannot assume an infinite series of causes, because an infinite series with no beginning involves a contradiction. Something has to have started off the whole process of the reception of existence. For every receiver there has to be a giver, so, for the first receiver there has to have been a first Giver who never received existence because He is existence itself. That first Giver is given the name God. Sooner or later we are left with the only logical explanation for the creation of all created things, namely, that there has to have been a first uncaused Cause from which all creation has proceeded. It is evident that if there ever had been nothing, there never could have been anything because nothing cannot give what it does not have. Nothing - Nothing. The fact that things exist, proves that there must have been a Thing to begin with who never needed to be created. A Thing - other things. God the Creator - creatures. No God = no creation.

References:
Laux., Catholic Apologetics; Carroll., Following Christ in the World.

* Please note that this text should be read in the context of the whole work and in recognition of the appropriate paragraphs of the Catechism of the Catholic Church highlighted in the index.