Deuteronomy 4:35 Unto thee it was shewed, that thou mightest know that the LORD he is God; there is none else beside him.
God showed great things to the nation of Israel. He won their battles for them. He spoke to them from the midst of the fire. They heard the voice of God and lived. He led His people and directed their paths. He showed His great power. He defeated Egypt, the most significant military force of their day. God did all that so that His people would know that He was indeed God.
This was not a coincidence. It was not something that some rebellious slaves pulled off. It was God. they needed to know that He was unlike any god of Egypt or others that they had heard of. They needed to know that the God of the Bible is the only God.