James 1:19-20

Wherefore, my beloved brethren, let every man be swift to hear, slow to speak, slow to wrath: For the wrath of man worketh not the righteousness of God.

Believers must remind ourselves to think fast, but speak slowly. While this is sometimes much more easily said than done. it is the command of God as part of our assignment to win lost souls. We cannot afford the luxury of saying everything we think because we sometimes think things that are not pleasing to the LORD. We live in fallen flesh, and sometimes that flesh wants to say what it wants to say. Our flesh does not want to honor the ways of the LORD. 

James reminds us that speaking out of anger and frustration does not work towards righteousness. We do not "let our light shine before men" (Matthew 5:16) when we speak out of anger. King Solomon wrote: "Be not rash with thy mouth, and let not thine heart be hasty to utter any thing before God: for God is in heaven, and thou upon earth: therefore let thy words be few (Ecclesiastes 5:2)." Not only does Solomon advise against speaking out of anger, but he admonishes us not to talk too much. Too many words will ultimately end up in sinful conversation. That conversation could potentially discourage someone who is lost from coming to Christ, or it could discourage a believer who is struggling. Regardless of the situation, believers need to talk less, listen more, and try not to speak out of anger.