The Greek text literally says: do not judge-separate [Greek: krìno].
Krìno means “I distinguish, I choose, I think, I consider, I estimate, I establish, I decide, I process, I accuse.” The root goes back to that of the Latin cèrno, “I separate, I distinguish.” Iindeed the base of this verb is to separate, to distribute, to winnow, to sift, to decide. In ancient times it concerned the action of making separate heaps of grain and straw: the ancient Babylonian word qaranu, in fact, means “to pile up grain and straw,” the Acadian word karawu means “to separate.” Krìno is the action of winnowing, which one can then extend semantically to the action of “passing judgment, thinking, considering, determining, deciding.”
Jesus forcefully invites us to respect this procedure, it is written literally: "Stop judging, that you may not be judged. For as you judge, so will you be judged, and the measure with which you measure will be measured out to you. Why? To Judge or separate is an an action that belongs to God, and only God has the expertise and the authority to do so. Judging is always taking God's place. Jesus warns us that just as when we judge our brothers and sisters we are taking God's place and doing something only God can do, others will do so the same to us. Indeed He points out that the same measure or gauge used to judge others will also be used to measure us. It is not blackmail, it is simply the procedure. It simply works that way for everyone.
But why, according to Evangelical procedures, is judging so bad?
When man judges he always does three abominable, deadly things.
First, he puts himself in God's place and uses human weights and measures, and by doing so carries out in a single instant a mental and spiritual gesture of absurd idolatry denoting deep, arrogant ignorance.
Second, while he judges, a man elevates himself to the position of a superior judge of justice towards his brothers and sisters, yet no man, in reality, is without debts and sins against love. When one judges, therefore that person performs an act of absolute hypocrisy and arrogance towards a brother, a sister, and God. Jesus emphasizes this concept by asserting: Why do you notice the splinter in your brother's eye, but do not perceive the wooden beam in your own eye?
The third abomination is the most dangerous one. When someone judges, that person uses the meter and measures set by men, without having the divine intelligence of mercy and the wisdom of forgiveness. The moment we decide to judge, and we do so with the meters and measures that we have decided on, in that very same moment we have also decided that one day those meters and measures will be used to judge our lives and our person.
Judging is an extremely dangerous and stupid action. The outcome of judgement is in no way advantageous, it simply generates separation and leads to sure condemnation.