Many, if not most, Christians believe God exists through faith. However, reason also has a role to play here. One point on faith and reason – “Reason and faith are not rivals but allies against irrational doubts, passion, prejudice, propaganda, fear, folly, fantasy and fallacy” (Handbook of Catholic Apologetics, p. 47). For instance, we have faith in Christ, but if we are challenged by a non-Christian, how would we explain our faith? We couldn’t just say, “Mommy and daddy told us” or “We read the Bible.” What about times when we question certain truths because we don’t understand them? Reason is a gift from God and it helps us to understand and explain truths received from God. Even Christ gave reasons to believe in Him by performing miracles, declaring prophecies that were later fulfilled and by rising from the dead to numerous witnesses. Therefore, reason has an important role in our Christian faith. Now, turning to your question, I will use a syllogism (structured argument) called the Kalam (Arabic: speech) argument.
Premise 1 – Whatever begins to exist has a cause for its coming into being. Take a moment and think about your existence. You had a beginning and a cause for your beginning; your parents. Your parents had a cause; your grandparents, who had a cause and so on. Apply this to animals, plants and all other living organisms and continue working backward in time until there is no matter, energy or space. If you continue working backward in your mind, you will arrive at absolute nothingness. Note: The earth is about 4.6 billion years old and the universe is about 14 billion years old (http://www.ips-planetarium.org/?age). Scientifically, this means the universe had a beginning.
Premise 2 – The universe began to exist (Better yet; matter, energy and space began to exist). Continue working backward. How did the universe come into being? Our best theory is Big Bang. Big Bang Theory was developed because certain physicists, including Father George Lemaitre, a Catholic priest, discovered that objects in space are constantly moving away from one another. By moving backward in time, the objects would move closer and closer together until they became a single, extremely dense mass called a “singularity.” Due to immense pressure, the singularity either exploded or rapidly expanded and, over millions of years, the matter, along with heat released from it, formed planets and stars. Continue working backward. How did the matter, pressure, energy and space come into existence without a being creating them?
Conclusion – The universe has a cause for its coming into being. We call this “cause” God. The universe needs something outside of itself to create it.
An atheist would probably offer the following rebuttals to this argument:
Rebuttal 1 – Couldn’t the universe, or at least the matter contained within it, be infinitely old? Answer: If the universe had an infinite past, we would never arrive at today. This is called infinite regression. Think about hovering over an infinite string of dominos. If the dominos have no beginning, you could never number a particular domino. You could never say, for instance, this is the three trillionth domino, because you would have an infinite number of dominos preceding it. The same concept applies to time. We could never say the universe is a trillion years old, because an infinite amount of time would have preceded it. Something outside of matter (i.e. immaterial) must have created space, matter, and energy.
Rebuttal 2 – You say God is infinitely old. Couldn’t the argument above (in rebuttal 1) be used for God. Answer: No. God is not material. He is pure spirit and, thus, does not need to be created. We cannot explain how God is eternal, but we can prove that all matter needs a beginning and that its beginning is outside itself.
Rebuttal 3 – Couldn’t the universe have been the result of random events? Answer: It’s important to understand that philosophers are not painting a picture of an empty space. They’re saying, at some point, there was absolutely no space. Therefore, nothing could have been spontaneously formed in space without an outside cause. Only an outside cause could create space, matter and energy.
Rebuttal 4 – What about evolution? Answer: Darwinian evolution (e.g. men evolved from apes) is pure theory. It is not fact. Even if it were fact, however, it does not disprove the existence of God. Even evolution needs a beginning. A thing, no matter how small, cannot create itself.
Less common rebuttals:
Rebuttal 5 – What about Multiverse Theory? Note: Multiverse theory states that there millions or billions of universes constantly in a state of flux. Answer: This theory does not answer the problem of an infinite past as discussed in Rebuttal 1. A multiverse still needs a creator.
Rebuttal 6 – What about Oscillating Universe Theory? Note: This theory states that there has always existed an expanding and contracting universe. Once a universe expands too far, it collapses and forms a singularity as discussed in Premise 2. This cycle (or oscillation) occurs repeatedly. Answer: Again, we encounter the infinite regression problem. The oscillating universe must have a beginning. (An additional resource: http://www.magiscenter.com/from-nothing-to-cosmos-study-guide/)
Thank you for your question!