One of the most common statements Jesus made, and he likely made it repeatedly, is actually a saying found throughout the ancient world. In Matthew 7:1 we find his familiar words: "Judge not, that you be not judged." Let me state, before I even explore this text, that Jesus is clearly not forbidding all forms of judgment. Criticism has a proper place. And the church, not individual Christians, is called upon to judge its own members.

Calvin
John Calvin says, "Jesus' words here are intended to cure a disease that is natural to us all. We have the tendency to flatter ourselves while passing severe censure on others. This vice provides us with a kind of strange enjoyment, for hardly anyone exists who is not tickled with the desire of asking about other people's faults. Yet we also acknowledge that it is an intolerable evil to overlook one's own vices while being critical of others."

So obvious is this human vice that even the ancient philosophers condemned this evil practice of excusing ourselves while condemning others. Calvin adds, "What is more, judging often includes another, worse sin, for most people who condemn others then think they have more freedom themselves to sin."

I learned, as a young minister, that this verse was one of the most commonly cited texts in all the Bible. In fact, it is one of those texts that many non-Christians know is somewhere to be found even if they are unaware of the biblical saying.

Images
Eugene H. Peterson's The Message paraphrases this text in a way that makes it come alive for modern readers:
     

Don't pick on people, jump on their failures, criticize their faults—unless of course you want the same treatment. That critical spirit has a way of boomeranging.

The common text says that the judgment we use will be used against us (cf. also Romans 2:1). To pass judgment is to assume God's authority. Mark 4:24 and Luke 6:38 speak about this very strongly.

Critical Person
Here is what I believe is going on in this context if the text is read properly. Disciples are called to live the life of the kingdom in a community that is known for mercy and compassion. Such a community should be known by its selfless concern for others. Such concern will correct others, but only in the spirit of discerning grace and mercy, not as a form of criticism or picking on people's faults.
The community is not one in which we spot people weaknesses and failures and point them out easily and flippantly, or as a course of routine practice. When this happens the "critical spirit" has a way of boomeranging!

One thing is for sure. No disciple has the right to decide the ultimate destiny of another disciple. Condemnation is not given to us but to God alone. The true context of mercy and compassion, in a healthy community of Christians, is found only in painful self-examination and ruthless correction of ourselves. Those who fail this requirement are almost always the same people who become critical and condemning toward other members of the community.