We all -or at least most- know that verse that says that what comes from the mouth is the overflow of the heart. As Matthew 15 puts it,
And he called the people to him and said to them, “Hear and understand: it is not what goes into the mouth that defiles a person, but what comes out of the mouth; this defiles a person.” Then the disciples came and said to him, “Do you know that the Pharisees were offended when they heard this saying?” He answered, “Every plant that my heavenly Father has not planted will be rooted up. Let them alone; they are blind guides. And if the blind lead the blind, both will fall into a pit.” But Peter said to him, “Explain the parable to us.” And he said, “Are you also still without understanding? Do you not see that whatever goes into the mouth passes into the stomach and is expelled? But what comes out of the mouth proceeds from the heart, and this defiles a person. For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander. These are what defile a person. But to eat with unwashed hands does not defile anyone.” (v. 10-20)That's puts a great weight on words. Even still, it does not denote cussing specifically. This is rather towards instruction and teachings; every plant that God has not planted will be rooted up (every teaching not from God will be destroyed); and if the blind lead the blind then both fall into the pit. Whatever instruction we subject ourselves to is the same way; it will be rooted up if it's not from God, and those we let ourselves be led by, if blind they be, will lead us into the same pit. Whatever instruction you receive extra-Biblically, be careful that it does not enter the heart without first being subjected to the Word of God.
Here in Romans, Paul mentions the great importance on the tongue;
...if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. (Romans 10:9-10)With the mouth, there is a matter of confession made unto salvation. It's not simply words that come from the mouth, but the extension of the heart. If our heart is driven by God, our words will be from God and, therefore, good (we will share the Gospel -literally Good News- with our mouth). If our heart is evil (or not of God), we can still lie about being righteous but even the profession of righteousness is unholy. The very saving power of Christ works in our heart, but it is not brought to fruition until it reaches our lips as confession.
We may confess without coming to salvation, but we cannot come to salvation without confession.
James has some of the stronger depictions of the importance of what we say;
If anyone thinks he is religious and does not bridle his tongue but deceives his heart, this person's religion is worthless. Religion that is pure and undefiled before God, the Father, is this: to visit orphans and widows in their affliction, and to keep oneself unstained from the world. (James 1:26-27)The essence of this is (along with the following passages), if we do not control our tongue, it will control us. Our belief system, our faith and our hope, is worthless if we do not "bridle [our] tongue[s]." Living in the world but not being of it is the only way we can be unstained from the world. We must wear the armor of God if we're to keep our inner-man from dirt, filth, bruising and beatings. Part of that armor, the shield of faith, mandates works in order to remain. These works, which reinforce our protection from the proverbial elements, are those such as visiting orphans and widows in their affliction; the works of faith which include consoling the mourning, giving to the needy, listening to the heavy-hearted, speaking to the empty, and the like. Of course, there are more works than these, these are but simple examples we may start with in everyday life.
For we all stumble in many ways. And if anyone does not stumble in what he says, he is a perfect man, able also to bridle his whole body. If we put bits into the mouths of horses so that they obey us, we guide their whole bodies as well. Look at the ships also: though they are so large and are driven by strong winds, they are guided by a very small rudder wherever the will of the pilot directs. So also the tongue is a small member, yet it boasts of great things. (James 3:2-5)James, the same fellow who stated that religion without temperance is worthless, says that the tongue is boastful, and that if we do what we say we are perfect in our ability to control our every action.
The imagery of the ships' rudders is one that strikes me quite profoundly; even if the winds are blowing eastwardly, a skilled sailor may use the sails and rudder to direct a ship westwardly. Likewise, if we are living a way of righteousness, the rudder must be controlled correctly; we must not allow this small member to turn us off-course.
How great a forest is set ablaze by such a small fire! And the tongue is a fire, a world of unrighteousness. The tongue is set among our members, staining the whole body, setting on fire the entire course of life, and set on fire by hell. For every kind of beast and bird, of reptile and sea creature, can be tamed and has been tamed by mankind, but no human being can tame the tongue. It is a restless evil, full of deadly poison. (v. 5-8)This is the specific direction the tongue would try to set us; Hell. The Truth shall set us free, we often hear, but how much easier a lie is to tell. Not only that, we don't have to keep track of what story has been told when only the truth is. It's our nature to lie, to blaspheme, to spew wrath or poison, because we have a tongue that craves it. Our throats are an open grave, and the venom of asps is under our lips; what comes from us is deadly unless we are able to control it. Antibodies come from venom. Fire burns forests, but a clean fire purifies. The tongue is a great weapon or a great tool, never to be underestimated, and the natural inclination is evil because it is "set on fire by hell."
With it we bless our Lord and Father, and with it we curse people who are made in the likeness of God. From the same mouth come blessing and cursing. My brothers, these things ought not to be so. Does a spring pour forth from the same opening both fresh and salt water? Can a fig tree, my brothers, bear olives, or a grapevine produce figs? Neither can a salt pond yield fresh water. (v. 9-12)This, the final passage i'll use, says that we inevitably do one of two things with the mouth; bless or curse. It is impossible to genuinely praise God with holy lips (see Isaiah 6 for reference) if we use them to curse. These curses are perpetuated by the same whose prayers are detestable to God. If we sing a shout of Hallelujah praise, or if we join the chorus of the Seraphim in a reverent, "Holy, holy, holy," or if we go into the closet and pray in silence, it churns the heart of God with repulsion if we spew venom outside of these scenarios. Sadly, i've watched people who commonly and unabashedly make vile remarks about people based on appearance or accent or vocabulary six days a week raise their hands in praise to their Maker within the walls of the church on Sunday, but it pains me to consider that their worship is met with detestation by God. They have cursed the image and likeness of God; how can they bless that from which these people were formed? It would be a lie to say i've never done this same thing, and it's shameful to admit, but it is growing less common with help from the experienced Pilot.
The first reaction to a frightening or disastrous scenario is perhaps the greatest insight to what the heart is filled with.
This is precisely why taming the tongue, be it against lies or slander or blasphemy (or, though unmentioned, cussing), is so important. And this is impossible by our own work. It can only be done with the help of God.