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Who is God?
Question 1. How do we know there is a God?
The most obvious question anyone asks a believer is "How do you know God exists?" It's a simple question that has both a simple and a complicated answer. First, the easy part: faith is believing in something or someone you cannot see or believing in what cannot be proven. In other words, faith depends on not having any evidence; otherwise, it would not be faith. So those who believe in God or in a supreme being are taking the word of others or just trusting their own instincts that such an almighty divinity does exist. Many people, whether they are Jewish, Christian, or Muslim, whether Roman Catholic, Eastern Orthodox, or Protestant, believe in God because they believe in the revealed Word of God, called Sacred Scripture or the Bible.
A more complicated answer is that Catholic Christianity does believe you can prove the existence of God. Reason can conclude that a supreme being exists and is necessary, but only faith (believing what cannot or is not known) tells us that there is but one God (monotheism). Judaism, Christianity, and Islam believe in only one God, and Christianity is the only of the three to believe that there are three Divine Persons in that one God. Human reason could never figure out the mystery of how there can be three persons but not three gods. Faith is needed to believe that doctrine, and some people never embrace that faith.
The fact that God exists can be known by reason alone, or it can be believed by faith. The ancient Greeks and Romans used philosophy (logic and reason) centuries before Christ to prove the existence of God or a supreme being. Saint Thomas Aquinas was a Catholic philosopher and theologian in the thirteenth century AD who used that same ancient reasoning to demonstrate the reasonableness of anyone "knowing" that God exists, regardless of whether or not they have faith. He showed that the existence of God can be proved by reason, but after that, one needed supernatural faith to believe the supernatural revelation about the nature of God (for example, the Trinity).