“But I say to you that if you are angry with a brother or sister, you will be liable to judgement; and if you insult a brother or sister, you will be liable to the council; and if you say, “You fool”, you will be liable to the hell of fire.” – Jesus (Matthew 5:22)
“With it [the tongue] we bless the Lord and Father, and with it we curse those who are made in the likeness of God.” (James 3:9)
How we carry one another in our hearts and on our tongues is a fundamental indicator of whether we are serious about following Jesus and living in the way of his mercy.
In his Directions on Spiritual Training, Dorotheus of Gaza (6th century), wrote:
What is more grievous than the sin of condemning one's neighbour? What else is so hateful and alienating to God? And yet a person comes to this great evil through something seemingly unimportant - from allowing himself a small censure of his neighbour. For when this is allowed the mind begins to leave its own sins without attention and notice the sins of its neighbour. And this leads to gossip, reproaches, speaking evil and, finally, pernicious condemnation. Yet nothing angers God more, nothing despoils a person and leads so surely to perdition as fault-finding, speaking evil and condemning one's neighbour.
– Paragraph 34
At times we not only condemn but bring our neighbor into contempt. For it is one thing to condemn, and another to bring into contempt. To bring into contempt means when a person not only condemns but also despises another, scorns him and turns away from him as from an abomination. This is worse than condemnation and much more pernicious.
– Paragraph 38
Those who want to be saved pay no attention to the failings of their neighbours, but always look for their own and make progress.
– Paragraph 39
Christians concerned about "bad language" would do well to worry less about the usual four-letter words (about which Jesus says nothing and the rest of the New Testament, very little) and worry more about the way they talk about, and the names they call, other people.
Becoming a People of God's Mercy and Delight, Part 1