We hear it constantly. People quote to us "judge not lest you be judged." Their intention is that we should not judge anyone else's sin but rather let them live however they want, and they'll take it up with God later. There are two flaws with this logic. First and foremost, they stop quoting that passage where it is convenient.
“Judge not, that you be not judged. For with the judgment you pronounce you will be judged, and the measure you give will be the measure you get. 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.
Notice that it begins with admonishing people not to look at anyone else's sins when they are living in sin themselves. Someone who is looking at pornography at night in his room alone should not be judging someone who they find out has been drunk the next day. It also reminds us to judge with mercy, and kindness. The 'measure we give will be the measure we get.' If we judge harshly? So will we be judged. If we judge with great mercy and understanding? So will we be judged. That's an important note....
But the most important thing is that at the end of this verse Jesus tells us "then you will see clearly to take the speck out of your brother's eye." What does that say? It says that judging is something we must indeed do. But as John reminds us, we are not to judge by appearance, but with righteous judgement. Why? Why would Jesus instruct us to judge others? Isn't sin just a personal thing? Isn't it between me and God alone? After all David said "against you alone have I sinned."
The thing to realize though is that we are Christians, and as such we have a theology of the Body of Christ. We are one body. Once we are baptized into Christ we are one living organism, one creation. We are the Church. St. Paul says it wonderfully when he compares us to body parts and
Think of sin as a cancer. Cancers always start with one cell type. They originate somewhere. The lungs for instance. Lung cancer is a horrible condition. It tears the lungs apart first. Yeah, if you are the lungs and I am the heart.. why do I care? It's your cancer. The problem with a cancer is it spreads. It doesn't just remain in one spot.. but eventually if untreated, begins to eat away at the entire body. In the south there is a saying for someone dying of cancer, "She's plumb eat up with it." It may have began with only one organ, one spot.. one member of the body.. but it hurts the entirety of it. Eventually causing other parts of the body to be cancer riddled too until it tries to kill the body.
The second is that sin is never a private matter. We may hide it. We may do it behind closed doors. We may only do it when no one else is around. It still affects our lives. It still changes us, and spreads. Look at our society itself. 2000 years ago it was unheard of to be an adulterer. Now we have entire reality shows dedicated to watching men and women cheat on each other. The more sin we accept, the more it spreads, the more it draws society (and its members) toward death. That's why Saint James tells us "confess your sins one to another." I believe sincerely that is also why Jesus gave his apostles the power and authority to forgive sins. Today we have confession, where that same authority is being used freely to forgive us when we fail; with the condition that we are trying to do better. Lord, I believe. Help my unbelief.