For a long time in history, people did not truly know God. However, the realisation of the truth of creation forced man to the knowledge of God yet, the form of God was hazy and confusing.
In the struggle to understand God, people thought there were many gods in charge of the world, and affairs of man. Man had thought no single entity had the overwhelming capacity to control the universe. This was the genesis of polytheism.
Polytheism held sway in the world for long. The ancient world, typified by Greek was a good example. They coined names for the so-called deities from the things they could see and perceive, like the sun, thunder, harvest, life etc. They worshiped these gods, and in fact, their lives revolved around them.
Other classical examples include Egypt, Babylon, Persia. In Nigeria, Igbos had Amadioha and Kamalu. The Yorubas had Ogun and Sango. The lingering effects of some of these gods even in today's modern world shows how strong their influence was.
But God, in his desire to let mankind know him did something. He did not want ignorance to continue. And so in Babylon (Ur), he called out a family to stand out from the crowd for the real thing - monotheism. That family was Terah's. Terah was the father of Abraham, Nahor and Haran.
When Terah died, God vested the calling in Abraham through whom a monotheist nation of Israel was born. Israel tried to project monotheism amidst the forces of polytheism which were overwhelming in her neighborhood. Sadly, Israel failed to an extent, and flirted with gods like Baal and Dagon.
But the failure, more than any other time began to take the backseat with the arrival of Jesus on the world scene right in Israel where he was born. He stepped his feet in a few cities in Palestine yet, the influence of his brief life which was catapulted by his cruel death shot the arrow of the true God into all corners of the world.
In a nutshell, Jesus taught the world of the only God, making his knowledge clear and enduring in an uncompromising way. No wonder he said, "He who has seen me has seen the Father."