The difference between a sinning unbeliever and a sinning believer is that one loves his sin while the other hates it. The believer who stumbles in his walk with the Lord regrets it, confesses it, wishes to never do it again and seeks to appropriate God’s power and grace to avoid it. He doesn’t consider how much he can sin and still be considered a Christian. Rather, he considers how he can avoid even the appearance of sin in the future
Jesus did not forbid judging.
"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 7:3-5)
This means to consider our own faults before nothing the faults of others. After we have honestly examined ourselves, it is then acceptable to note the fault of another, and then we are to help them turn away from their sin.
"Do not judge by appearances, but judge with right judgment" (John 7:24)
When we are called upon by the circumstances of any given situation to make a judgment about spiritual matters, the judgment we make cannot be based on our own standards. Personal opinion has no part in spiritual matters and all opinion is equally useless. The standard by which we will be judged is the words of Jesus.