Does calling God a liar seem like a good idea to you?

It just doesn't seem like a good idea to me and yet millions of people do it every day.  Why do you think that is? 

1 John 5:10(NKJV)

He who believes in the Son of God has the witness in himself; he who does not believe God has made Him a liar, because he has not believed the testimony that God has given of His Son.