Therefore you have no excuse, O man, every one of you who judges. For in passing judgment on another you condemn yourself, because you, the judge, practice the very same things. We know that the judgment of God rightly falls on those who practice such things. Do you suppose, O man—you who judge those who practice such things and yet do them yourself—that you will escape the judgment of God? Or do you presume on the riches of his kindness and forbearance and patience, not knowing that God’s kindness is meant to lead you to repentance? But because of your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when God’s righteous judgment will be revealed.
He will render to each one according to his works: to those who by patience in well-doing seek for glory and honor and immortality, he will give eternal life; but for those who are self-seeking and do not obey the truth, but obey unrighteousness, there will be wrath and fury. There will be tribulation and distress for every human being who does evil, the Jew first and also the Greek, but glory and honor and peace for everyone who does good, the Jew first and also the Greek. For God shows no partiality. Romans 2:1-11.
The reason that judging others is so bad is that we are denying others the mercy we enjoy. We are not children of God because we are better than others. He enables us to become His children because we depend on His grace. This is a grace that belongs to the most recalcitrant sinner if only he would turn and believe.
This is what set Jesus and the self-righteous Pharisee apart. The Pharisee was given light to use to draw others to God. Instead, the Pharisee used it to expose the sin of others and repell them. They became mean and arrogant, embellishing their credentials while finding fault with the weak.
Are we any different? Do we disdain the sick we were sent to heal? Are we more interested in keeping ourselves unspotted from the world than being touched by the pain of those who suffer in the dark? Are we too good to serve the broken? We need to remember that our own salvation was purchased at a great cost, and that it was not for us alone.