The existence of God.
St. Thomas Aquinas was a Dominican friar who lived in the year 1200's. He was born to a noble family in Roca Seca, the kingdom of Sicily, Italy. Although his vocation was strongly opposed by his family, he pursued it. Thus, we benefit from his works in the fields of philosophy and theology for which the Church was made him one of the doctors of the Church.
His acclaimed work, the Summa Theologiae (Summa Theologica) is regarded as a "manual for beginners in theology" and as a "compendium of all the main theological teachings of the Church."
Is there a God?
Is there a God who exist?
Do God exist?
Let the Quinque Viae or the Five Ways of the Summa Theologica answer those questions.
The first way is the argument of the Unmoved Mover. This tries to explain that God must be the "cause of motion" in the universe. The universe is in motion but it could not have set itself in motion nor there can be infinite, perpetual or continuous motion. A being outside the universe had to set everything into motion, thus, according to the Summa, this Unmoved Mover is what we call God.
The second way is the argument of the First Cause. This tries to show that God must be the cause or the "creator of the universe". Now, everything in the universe is created by something else but these cannot go on infinitely. There must be a first cause and St. Thomas tells us that the "First and Uncaused Cause is what we call God.
The third way is the argument from Contingency. This tries to explain that the universe is created by God who is needed by others and "non-dependent on anything or anyone else". The whole universe is contingent for the universe and its contents to have existed, certain factors must have been in place. But there cannot be an infinite chain of contingencies. There must be something or someone who relies on nothing else in order to exist. Something or someone who created the conditions giving rise to the universe and all of its contents. Still from the Summa, this non-contingent being is what we call God.
The fourth way is the argument from Degree. This tries to explain that God is the "cause of all goodness and perfection." Due to varied judgements, there are varying degrees of perfection so one can say something is better than the other or not. But in order to come up with such judgement, there must be an ultimate standard of perfection. St. Thomas categorically tells us, this pinnacle of goodness and perfection is whom we call God.
The fifth way is the argument of design. This claims that everything in the universe was "made" or "has a purpose" caused by God and follows the laws created by God. We are aware that everything in nature is orderly and systematic including the living and non-living things. Hence the universe having governing laws like the laws of physics among others proves the existence of an intelligent being who created these. Thus the Summa of St. Thomas states that this Intelligent Designer is whom we call God.
In conclusion, truly, God do exists.