It is written that death and life are in the power of the tongue, and again: Many have fallen by the edge of the sword, but even more because of the tongue.
The Lord says the same thing: They have taught their tongues to speak lies. They have taught!
You see, the tongue becomes accustomed to telling lies. It tells lies even when you do not want it to.
It is like a wheel: if you spin it once, after that initial impetus its own shape and roundness or what you might call its natural instability makes it go on turning.
And it is the same with our tongues: once started they run on of their own accord in the way that is easiest for them.
You have one thing in your mind, but sometimes out of habit the tongue chooses another.
What is to be done? You see what a balanced judgement must be made before the tongue is allowed to say anything!
For it does not in fact wag of its own accord; there is one within who wags it. There is within us a certain power which moves both itself and the members that serve it.
Let the one in control be good and with the help of grace that person can overcome any bad habit whatever. Let the servant be good and the service will be peaceful.
The soldier has weapons but if he does nothing neither do they. So too among our members our tongues are our souls’ weapons.
Scripture calls the tongue a restless evil. O restless member! Who made this evil if not a restless person? Do not be restless yourself and this evil does not exist. Do not set it going and it will do nothing on its own.
It is not a spirit to move of its own accord. It is merely a body and lies still. It will not wag if you do not wag it. When you do use it, be careful how you do so.
[…] What an impious tongue! You have despised the Creator and respected the creature! Oh that restless evil, full of deadly poison! We use it to praise our God and Father – God and also Father, God by nature, Father by grace – then we use it to call down curses on other people made in God’s image.
Be careful, my friends, with what you are carrying about with you. But of course I should say, what we are carrying about with us, for I am a man just as you are.
Augustine of Hippo (354-430): Sermon 14A.2-3 (CCL 41:219-220); from the Monastic Office of Vigils, Saturday of the 28th Week in Ordinary Time, Year 1.