St. Thomas Aquinas was one of the greatest philosophers the Church has ever seen. He tried to show the harmony between faith and reason, and between Christianity and philosophy. Aquinas’s views have been very influential.
Aquinas gave five classic “proofs” for the existence of God. Here they are taken from his massive Summa Theologica (Part 1, Question 2, Article 3).
1 – FIRST MOVER: Some things are in motion, anything moved is moved by another, and there can’t be an infinite series of movers. So there must be a first mover (a mover that isn’t itself moved by another). This is God.
2 – FIRST CAUSE: Some things are caused, anything caused is caused by another, and there can’t be an infinite series of causes. So there must be a first cause (a cause that isn’t itself caused by another). This is God.
3 – NECESSARY BEING: Every contingent being at some time fails to exist. So if everything were contingent, then at some time there would have been nothing — and so there would be nothing now — which is clearly false. So not everything is contingent. So there is a necessary being. This is God.
4 – GREATEST BEING: Some things are greater than others. Whatever is great to any degree gets its greatness from that which is the greatest. So there is a greatest being, which is the source of all greatness. This is God.
5 – INTELLIGENT DESIGNER: Many things in the world that lack intelligence act for an end. Whatever acts for an end must be directed by an intelligent being. So the world must have an intelligent designer. This is God.