It will be helpful to define religion before we attempt to classify any belief system, such as atheism, as a religion. These definitions come from dictionary.com:
1. A set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs.
2. A specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects.
3. The body of persons adhering to a particular set of beliefs and practices.
4. The practice of religious beliefs; ritual observance of faith.
5. Something one believes in and follows devotedly; a point or matter of ethics or conscience.
According to these definitions, atheism can indeed be considered a religion.
In the Bible we discover that, when God created humankind, He designed an awareness of His existence into human nature. This awareness is not full knowledge or understanding of God; it is simply the built-in knowledge—we might call it “instinct”—that there is an eternal being, God. Paul explained it quite clearly in Romans 1:19: “For what can be known about God is plain to [unbelievers], because God has shown it to them” (ESV). Everyone—believers and unbelievers alike—knows there is a God, even though they may deny that knowledge. History gives us an example. The Athenians had an altar dedicated to a God they knew existed, did not know, and whom they knew they must worship (Acts 17:23). The Athenian altar “To the Unknown God” exhibits proof all human beings know there is a God.
God designed human beings to worship Him. When a person does not worship God, he or she will worship anything-but-God. That is what atheism is. When a man rejects God, he creates false gods to receive his worship. Mankind creates gods in their own sinful image, e.g., the Greek and Roman gods who were immoral, mendacious, venal, petty, and cruel. In more modern times, there is the Muslim god who commends dying (including by suicide) in a religious cause as the highest form of worship; and the secular humanist god who commends infanticide and euthanasia as the greatest expressions of personal freedom. Man has created gods of the stars, the planets, the sea, the wind, rivers, plants, and animals; has turned his ancestors into gods; has worshiped spirits who supposedly exist in animate and inanimate things; and has imagined that he can become a god himself.
Even the atheist creates a god he can worship. Psalm 14:1 states, “The fool has said in his heart, ‘No, God’”—the literal rendering of the Hebrew text. The fool doesn’t deny the existence of God; he rejects the authority of the one true God, replacing God with himself. In other words, he becomes his own god. He says, “No God for me!” meaning “no God but me.” J. J. Stewart Perowne writes of Psalm 14:1, “There is here rather a practical than a theoretical atheism; not so much a denial of the being of a God as a denial of His moral government of the world” (The Book of Psalms, Zondervan, 1966, 1:183–84).
Every human being knows, because of the way God designed human nature, that there is a God who must be worshiped, a God who requires submission and dependence. But the atheist refuses to submit to God and therefore rejects God. He hides his rebellion with the lie—to himself and others—that there is no God. Atheism is not disbelief in God but the rejection of God.
Is atheism actually a religion? The answer is “yes.” The atheist worships—he cannot help it—because God designed the necessity to worship into human nature. The atheist, like all who reject the one true God, creates an idol—in his case, it is an idol of himself to satisfy both the inner knowledge that there is a God and the inner necessity to worship that God. The atheist creates a religion of trust in himself that meets one or more of the above definitions of religion.