The Chou had tried to overthrow the Shang kings for some time. The Chou needed the nobles on their side. The nobles were torn. Obviously, there was going to be a battle. The nobles did not want to support the losing side.
The Chou did something very clever. They told the nobles that the gods had decided that the Chou had the right to rule. The Chou called this the Mandate of Heaven. The Chou explained that the gods had said they would only let the Chou rule as long as they were good rulers. If they became selfish, like the Shang kings, and thought of themselves first, the gods would appoint a new ruler again.
The nobles probably did not believe this fairy tale. But they were tired of the constant fighting that broke out between the Shang and the Chou. Most of the nobles knew life would continue pretty much the same under either ruler. What was important was to stop the warring. The Mandate of Heaven sounded like a good idea.
The Mandate of Heaven created a justification system. The Mandate either said or implied three major things.
(1) The right to rule is granted by the gods. This gave the ruler religious power.
(2) The right to rule is only granted if the ruler cares about his people more than he cares about himself. This gave the ruler secular power, or power over the people, and the right to decide what is good for the people, because the ruler must care about the people, or the gods would remove him as ruler.
(3) The right to rule is not limited to only one dynasty or family. A dynasty can be replaced. This justified rebellion. When a new leader leads a successful rebellion, he must have the support of the gods, or he would not be allowed to rule, because it was the gods who chose the rulers.
The nobles agreed that it was true that the Shang had become selfish. Perhaps the Chou would be a better choice. They joined the Chou in rebellion, and the Shang were deposed (no longer kings.) Since this rebellion was successful, the Chou obviously had the right to rule.