There are things which it is our duty to cover in silence. We are told nowadays that everything ought to be expressed if we are truly "honest" and "open."
Proverbs 11:13 says, "He who goes abroad as a talebearer reveals secrets, but he who is trustworthy in spirit keeps a thing hidden."
Jesus sometimes refused to reveal the truth about Himself, even when it would have seemed to us "an opportunity to witness." He did not always answer questions. He did not always say who He was. He told some of those He healed to tell no one about it.
"For every activity under heaven its time...a time for silence and a time for speech" (Eccl 3:1,7 NEB). "A man of understanding remains silent" (Prv 11:12 RSV).
Lord, deliver me from the urge to open my mouth when I should shut it. Give me the wisdom to keep silence where silence is wise. Remind me that not everything needs to be said, and that there are very few things that need to be said by me.