There is no other entity or person with whom we are related about which we can say that. Everyone else, every other organization or government we cannot trust to do what they say they will do all of the time. They will make mistakes, fail, change their mind, whatever. God does not.
He is the constant. His behavior is constant. We can depend on Him. We can rest in the fact that He will be faithful to who He is and what He promises to do.