Throughout the Bible, we are warned about the importance that God places on our verbal utterances. In the Torah, the Jews were solomnly warned that upon the words of their mouths, they would be held accountable.
If a woman vows a vow to the Lord and binds herself by a pledge, while within her father’s house in her youth, and her father hears of her vow and of her pledge by which she has bound herself and says nothing to her, then all her vows shall stand, and every pledge by which she has bound herself shall stand. But if her father opposes her on the day that he hears of it, no vow of hers, no pledge by which she has bound herself shall stand. And the Lord will forgive her, because her father opposed her.
If she marries a husband, while under her vows or any thoughtless utterance of her lips by which she has bound herself, and her husband hears of it and says nothing to her on the day that he hears, then her vows shall stand, and her pledges by which she has bound herself shall stand. But if, on the day that her husband comes to hear of it, he opposes her, then he makes void her vow that was on her, and the thoughtless utterance of her lips by which she bound herself. And the Lord will forgive her Numbers 30:3-8
The first thing that stuck out to me in this was that it was irrelevant whether or not you "meant" to put yourself under a vow or an oath. The only thing that mattered was that you were bound by law simply by a thoughtless utterance of your lips.
The wise Solomon also exhorted us to pay attention to what we say with our lips, knowing that God is in heaven, and takes us literally.
Be not rash with your mouth, nor let your heart be hasty to utter a word before God, for God is in heaven and you are on earth. Therefore let your words be few. For a dream comes with much business, and a fool 's voice with many words.
When you vow a vow to God, do not delay paying it, for he has no pleasure in fools. Pay what you vow. It is better that you should not vow than that you should vow and not pay. Let not your mouth lead you into sin, and do not say before the messenger that it was a mistake. Why should God be angry at your voice and destroy the work of your hands? Ecclesiastes 5 2-6
Nothing has changed on this subject in the New Testament. Christ reiterates the importance of watching your tongue in the Sermon on the Mount, as well as in discourse with the Jewish leaders. His words are profound, expecially in chapter 12:
But I say to you, Do not take an oath at all, either by heaven, for it is the throne of God, or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King. And do not take an oath by your head, for you cannot make one hair white or black. Let what you say be simply ‘Yes’ or ‘No’; anything more than this comes from evil.“Swear not at all” Matthew 5:34-37
I tell you, on the day of judgment people will give account for every careless word they speak, for by your words you will be justified, and by your words you will be condemned. Matthew 12:35-36
James tells us that if a man can control his tongue, then he is perfect, and able to control the entire body:
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.
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. 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. James 3:2-10
Therefore, brothers and sisters, let us focus on taming our tongues. It is of the utmost importance!