Judge not, that you be not judged. (Matthew 7: 1)
Jesus said to his disciples: “Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and with the measure you use it will be measured to you. Why do you see the speck that is in your brother's eye, but do not notice the log that is in your own eye? Or how can you say to your brother, ‘Let me take the speck out of your eye,’ when there is the log in your own eye? You hypocrite, first take the log out of your own eye, and then you will see clearly to take the speck out of your brother's eye. (Matthew 7: 1-5)
The kind of judgement that Jesus forbids is that of judging individuals, not that of their behaviour. In fact, we must judge people’s behaviour, both our own and the behaviour of others, in order to constantly check whether it is in line with God’s commandments. Hence, we must refrain from judging sinners, but we must judge their sins in order to fight against temptation and our own even tendencies from their very beginning and without delay.