Luke 1:22

But when he did come out, he was unable to speak to them. They realized that he had seen a vision in the temple and he kept making signs to them, and remained mute.

Zacharias had a believing problem! The angel of the Lord just appeared telling him he was to have a child; however, he spoke doubt.  Jesus reminds us to only speak in faith and not doubt.  If we have thoughts of doubt coming to our minds, we must cast them down.  Our lives will be defeated unless we are just as careful with what we say as our prayers.