There is a long discussion by Jesus in Matthew 12 about how we talk and the things we might say. At the center of the discussion is questions about the Spirit that is behind Jesus works. Both Jesus and His opponents acknowledge that there is a spiritual power behind His ministry. His opponents said that Jesus is working by the power of the devil. They say,

… “This man does not cast out demons, except by Beelzebul, the prince of the demons.”
Mtt 12:24

Jesus went on to show that this was not a reasonable proposition, the idea that Satan would oppose his own destructive work. Then He went on to warn of ominous future consequences.

“31 Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven men. 32 Whoever speaks a word against the Son of Man, it will be forgiven him; but whoever speaks against the Holy Spirit, it will not be forgiven him, neither in this world, nor in that which is to come.”
Mtt 12:31-32

A scary prospect to say the least.

Then Jesus makes some general observations about human speech.

“34 You offspring of vipers, how can you, being evil, speak good things? For out of the abundance of the heart, the mouth speaks. 35 The good man out of his good treasure brings out good things, and the evil man out of his evil treasure brings out evil things.”
Mtt 12:34

Again here Jesus is commenting that if we store evil and wicked things in our hearts, they will without fail tend to come out of mouths.

Then Jesus again forecasts what will happen to us.

“36 I tell you that every idle word that men speak, they will give account of it in the day of judgment. 37 For by your words you will be justified, and by your words you will be condemned.”
Mtt 12:36-37

Jesus tells us that there IS a record of all of our activities and all of our speech. This is testified to more than one Scripture.

“I saw the dead, the great and the small, standing before the throne, and they opened books. Another book was opened, which is the book of life. The dead were judged out of the things which were written in the books, according to their works.”
Rev 20:12

More than that, we will have to give an account for ALL of those stupid and senseless things we may say from time to time. That should be a scary thought to all us. Who of us would not be caught in such a net.

These things are not meant as idle speculations of what might happen

These are prophecies springing from that same Holy Spirit of God that Jesus opponents were slandering. Prophetic statements of the nature of things in our world. Statements, which if we believe them, and we should, SHOULD change our conduct and our speech.

Prophecy is NOT an optional extra! Believing what God says in prophecy does matter! It is an ethical matter.

Scriptures are from the World English Bible (WEB), a copyright free revision of the original ASV American Standard Version 1901