Theos is the Greek word for God. It is from this word that we get our English word theism. Theism is belief in God. A person who believes in God is called a theist. On the other hand, atheism is disbelief in God. (In Greek, "a" means "not" or "without.") Atheists are those who do not believe in God.
You may wonder how anyone could ever be an atheist. "The heavens declare the glory of God; and the firmament shows His handwork" (Psalm 19:1). God's "invisible attributes are clearly seen, being understood by the things that are made" (Romans 1:20). The design of the Universe and everything in it (including humans, animals, and plants) bear witness to an intelligent Designer. How could anyone ever say that God, the grand Designer, does not exist?
In truth, no one can logically defend the statement, "I know that there is no God." Why? Because humans do not know everything. We do not know every fact that there is to know. For a person to "know" there is no God, he would have to know everything; otherwise the one thing he does not know could be the fact that God exists. So, in reality, though people may say, "There is no God," they could never prove such a statement.
Still, why is it that millions of people on Earth disbelieve in their Creator? Why do they doubt His existence? There are many reasons for disbelief. In fact, if you polled 100 atheists and asked them why they do not believe in God, you would get a variety of answers.
Some people do not believe in God because their parents don't believe in God. Many children simply believe what their parents say without really thinking for themselves. But believing something only because your parents believe it is not reasonable. Some parents may teach their children that two plus two is five, or that Abraham Lincoln was the first President of the United States (rather than the 16th), or that murder is a good thing. But such teachings are wrong. Likewise, atheism is wrong. God exists even if parents say otherwise.
Some people disbelieve in God, not because their parents and mentors have taught them that God does not exist. Rather, they do not believe in God because they have never heard the positive, powerful proofs of God's existence. Young people who never wonder how everything in the Universe got here, or how human intelligence demands that a greater Intelligence (God) must exist, may grow up to become atheists. For this reason, Christians need to be ready and willing to give people sound arguments for God's existence—arguments such as design demands a Designer.
Many people are atheists because they believe "all smart people are atheists." If intelligent teachers say that God does not exist, then God must not exist, right? Wrong! First, not all "smart" teachers or scientists are atheists. Many thousands, perhaps millions, of brilliant scientists around the world are theists, not atheists. Second, a person should never accept something as true just because others do. Even if everyone on Earth were atheists, that still is no excuse to be an atheist. Truth is truth regardless of who or how many disbelieve it. Third, the Bible says, "The fool has said in his heart, 'There is no God'" (Psalm 14:1; 53:1). Some atheists may be highly trained and educated, but if they do not believe in the Creator of the Universe, they are foolish, not smart.
A number of people choose to be atheists because they want to be their own boss. That is, they do not want to accept the idea that they may be accountable to a higher Being. Whereas Christians live life on Earth in view of the fact that they must act according to God's will, many atheists like the idea of not having to answer to a higher power. They do as they please. If there is no God, then there is nothing really wrong with any actions such as cursing, lying, and cheating. The fact is, however, everyone is subject to the Creator, whether they want to believe it or not.
These are only a few of the reasons why people become atheists. Others choose to reject God because of pride, or because of the suffering they see in the world, or because they know insincere Christians who act as if there is no God (though they profess to believe in Him). Yet, when all is said and done, there has never been and never will be a good reason to reject God. No one can reasonably be an atheist, but anyone can sensibly choose to believe in God and become a Christian (Revelation 22:17; John 3:16; Mark 16:16; Acts 2:38).