Answer
The oldest religion in the world is the worship of the One True God, as stated in Genesis 4:26, “At that time people began to call on the name of the Lord.” The name they called upon was Yahweh (Latinized as “Jehovah”). The fact that they “began” to call on this name indicates a change in society—for the first time, people were organizing and identifying themselves as worshipers of God. This happened during the lifetime of Enosh, Adam’s grandson through Seth, about 250 years after the first couple was expelled from Eden.
About that early form of worship we have no information concerning formal instructions from God or set traditions on how the people carried out their worship. We can assume that sacrifices were involved, since Cain and Abel had understood the need for individual sacrifices a generation prior (Genesis 4:3–4). All Moses tells us of that first “religion” is that they knew God’s name and they called upon Him.
Satan corrupts and divides. It wasn’t long before the pure religion of calling upon God’s name was corrupted by idolatry and divided into literally hundreds of religions. By Noah’s day, the name of God had been all but forgotten, and “every inclination of the thoughts of the human heart was only evil all the time” (Genesis 6:5). The next time we read of anyone calling upon God’s name is in Genesis 12:8; that’s when Abraham “built an altar to the Lord and called on the name of the Lord.”
The oldest pagan religion for which we have evidence of an organized following is Egyptian. The Egyptian culture with its many gods was already well established by the time described in the latter part of Genesis and the book of Exodus. Abraham had dealings with a wealthy, growing Egypt and its pharaoh (Genesis 12:10ff).
At the time of Moses in the 15th century BC, the Lord once again revealed His name (Exodus 3:14) and codified religion for the Israelites. Required as part of calling upon the name of Yahweh was the forsaking of all other gods (Exodus 20:3–4). In the midst of a pagan, polytheistic world, the monotheism of the Hebrews stood out as a light in a dark place.
The religion we know as Christianity is really a continuation of God’s plan for the Israelites. The gospel is “the power of God that brings salvation to everyone who believes: first to the Jew, then to the Gentile” (Romans 1:16). So, the history of the world involves a cycle of God’s revealing Himself to mankind, man’s falling away from that knowledge, and God’s restoration of the truth. Tracing that line of truth all the way back to Genesis 4:26 and the promise of Genesis 3:15, one could even say that the worship of God in Christ is the world’s oldest religion.
