God proved Himself in lots of ways to the Fathers of faith, to Abraham, to Isaac, to Jacob. He proves Himself to their descendant when they were in Egypt. He performed wonders that the Gods of Egypt could not control.
When they left Egypt for cannan, He performed lot of wonders on their way. He parted the red Sea for them, He fed them, He dried the river of Jordan before them, He pulled down the wall of Jericho and many more.
The Lord is still proving Himself to us now and He will continue to do because He is the only True God.
You can watch the answer of Bro Eliseo from the link below.