Lesson 2 · 12 min
Can We Know God Exists? Faith and Reason
How natural reason can lead the mind to affirm God's existence, and how this truth is confirmed and completed by revelation.
Introduction
What this is
This lesson explains how unaided reason can establish that God exists, and how revelation completes what reason alone cannot reach.
Why it matters
Many people wrongly assume faith is a blind leap; understanding the partnership of faith and reason equips you to answer that charge.
Reason's Testimony
St. Paul teaches that 'the invisible things of him, from the creation of the world, are clearly seen, being understood by the things that are made; his eternal power also, and divinity' (Romans 1:20). This means that human beings, using unaided reason, can look at the order and existence of the universe and conclude that a Creator exists — a truth the Church calls a 'preamble of faith.' Psalm 19:1 declares, 'The heavens shew forth the glory of God: and the firmament declareth the work of his hands.' The Church has never taught that faith is a blind leap; rather, natural theology and revealed theology work together, each confirming the reasonableness of belief.
The Five Ways of Aquinas
St. Thomas Aquinas, in the Summa Theologiae (I, q. 2, a. 3), offered five classic arguments (the 'quinque viae') for God's existence, arguing from motion, efficient causality, contingency, degrees of perfection, and the governance of the world toward an end. The argument from motion, for example, observes that nothing moves itself from potency to act; there must be an unmoved first mover, which all men call God. The argument from contingency notes that everything that exists could fail to exist, so there must be a necessary being that grounds all contingent reality. These are not merely medieval curiosities; they remain serious philosophical arguments discussed by philosophers today, believers and non-believers alike.
Beyond Reason: Revelation
While reason can establish that God exists, it cannot by itself disclose the inner life of the Trinity or God's plan of salvation; these are known only because God has freely revealed them (CCC 50). Hebrews 1:1-2 says God 'spoke in times past to the fathers by the prophets... last of all, in these days hath spoken to us by his Son.' Faith, therefore, is not irrational but supra-rational: it builds upon what reason can establish and adds truths reason alone could never discover, truths we accept on the authority of God who reveals them. This is why an apologist can honestly say both that God's existence is demonstrable and that the Trinity is a mystery accepted by faith.
Knowledge check
Answer each item, then read why the answer is correct.
According to Romans 1:20, how can God's existence be known?
True or False: Aquinas' argument from motion concludes there must be an unmoved first mover.
Which verse teaches that God spoke fully and finally through his Son, beyond what the prophets revealed?
Explain why the Church teaches that faith is not opposed to reason, even though some truths (like the Trinity) exceed unaided reason.
Apologetics practice
Choose the response you would give, then see how it holds up.
Objection
“"Believing in God is just a blind leap of faith with no rational basis whatsoever."”
Objection
“"Science has replaced the need for God as an explanation for the universe."”
Key points
- Romans 1:20 teaches God's existence can be known from creation by reason alone.
- Aquinas' Five Ways argue from motion, causality, contingency, perfection, and finality.
- Revealed truths like the Trinity exceed reason but do not contradict it.
Lesson summary
Main points
- Reason alone, from creation, can establish that God exists (Romans 1:20).
- Aquinas' Five Ways give classic philosophical arguments for God's existence.
- Revealed truths exceed reason but never contradict it.
Important verses
Romans 1:20 · Psalm 19:1 · Hebrews 1:1-2
Important quotations
“I do not seek to understand in order that I may believe, but I believe in order to understand.”
St. Anselm of Canterbury — Proslogion, Prologue (c. 1078)
Quick defense answer
Reason can establish God's existence from creation (Romans 1:20); revelation then adds truths reason alone could not discover, without contradicting it.