God is not man, that he should lie, or a son of man, that he should change his mind. Has he said, and will he not do it? Or has he spoken, and will he not fulfill it? –Numbers 23:19
This verse says a lot about man while saying a great deal about God. By informing us what God is not like, we are informed what man is like.
First, man is a liar. It is in our nature to mimic the Devil. He is our first spiritual father when we arrive on this earth. He is our father until we individually become adopted by our Heavenly Father through Jesus Christ. God is not a liar like we are.
Second, man changes his mind. We are never completely consistent. The truth is the only set of ideals that consistently fits together, and man is not born with this set of heavenly ideals.
Third, man does not always do what he says he will do. Only God always keeps his word.