Bhagavad Gita - chapter 4, verse 7

Yada yada hi dharmasya glanirbhavati bharata
Abhyutthanamadharmasya tadatmanam srjamyaham

"Oh, Arjuna, when there is a decline of righteousness and sins are increasing, I descend and remove sins from the world."

When people are engrossed in their worldly nature and feel that their material bodies and material gain are the ultimate goals, they do not want anything. They only want to eat, drink and be merry. In this stage, they totally deviate from the truth. They feel that they are scientists and are inventing many things, that they are doctors and are helping the nation in many ways and that they are rich. They do not want God. But they do not know that without their inhalation, which is constantly being pulled by the Soul, they are all dead bodies. The indwelling Soul alone is doing everything.

When the general mass of people arc engrossed in their worldly nature, an agent of God descends into the world and, according to the needs of that time, teaches people scientifically that a body-car is useless without the Soul driver. Without Soul, there is no eating, drinking and merry-making. He also teaches that the power of God is abiding even in food matter. Without this power, human beings have nothing. The agent of God is transformed spiritually into forms by which scientific persons can perceive Soul in scientific ways. Then the people follow the new agent of God, and in their every inhalation, they feel the power of God. They feel He loves them much, so they will love Him. They will seek Him and will remain indebted to Him. If they do not follow His instructions 100 percent, they cannot get real peace, joy and bliss.