"Casting down imaginations, and every high thing that is exalted against the knowledge of God, and bringing every thought into captivity to the obedience of Christ;" (ASV)
We've all heard the old adage, "If you don't have anything nice to say, don't say anything at all." While this is certainly sound advice, a quick study of the Bible will show that God actually expects quite a bit more from His followers.
Certainly it is important to remember that our words can cause us great harm. "Whoso keepeth his mouth and his tongue Keepeth his soul from troubles." (Proverbs 21:23 ASV) We must learn to keep our tongues under control. This includes not only saying spiteful things to other people, but also lying, gossip, and impolite language. It isn't ok to say mean things behind someone's back, it's not ok to tell little white lies, and it's certainly not ok to spread rumors you have heard. All of these activities can hurt those we are called to love. But simply holding your tongue is not enough.
Not only must we be the masters of our words, but our thoughts must be under our control as well. "And Jesus knowing their thoughts said, Wherefore think ye evil in your hearts?" (Matthew 9:4 ASV) God knows not only that which comes out of our mouths, but also that which we think in our innermost being. He knows that in order for us to come into line with His perfect will, we must must free ourselves from the bitterness, hatred and wickedness of the world around us, and that means cutting it out at the root. It is not enough to love others with our words and deeds. We must also learn to love them with our hearts. We must learn to see the world as God sees it, with love and compassion.
So the next time you find you need to hold your tongue to keep from saying something that might hurt someone else, go one step further. Pray that God will take that hurtful thought and banish it from your life.