What Christ tells the Pharisees, and indeed all of us, here is God has a level of scrutiny of our speech far above anything we could imagine. The Pharisees have blasphemed Christ.
We must guard against idle words. Every word must be under control, and we must give great attention to our speech. To speak without self-scrutiny is to open up a channel out of our hearts that affects the whole person. Gossip can be carelessly uttered, but we must speak is love and with concern for the reputation of others at all times. Small inaccuracies can become small lies and these can turn into bigger lies and integrity can leak away. Complaining can become habitual and we become incapable of giving thanks to God. When we do, the complaining mouth makes it impossible for our thanks to be sincere. Our complaints to men are declaration against God, for he is in control of all our circumstances. Words will stand as evidence against us. What horror we will experience when we realise how much harm we have done! Christ has died to save. Every extra evil word adds to that record of sin which he had to bear away.
What is the point of restraining the tongue when the same evil is still in the heart? There is, we are told here, an extra judgment for speaking, even though the heart is already corrupted. To speak is to go a step further, and to bring out of the heart what lies hidden in it. We bring it out because we think it has such value that others ought to know about it. To speak is to comment on our own thoughts.