This phrase “Who am I to judge?” is a creche and it is very misleading and can be wrongly used among Christians quoting from the scriptures, causing mishaps and undesirable life application and usage because of the contexts they are in. I am concerned that we can be constrained to rebuke sins or correct them and speak the truth because of this phrase, and it does sound humble and making sense on the surface. That’s the problem, because it is sneaky and in reality it contradicts the scriptures among themselves, because they are used in different contexts. I have explored those instances when this phrase is used, and looked at the context they are in.
We are not to judge one another on disputable matters like eating food
Romans 14:1–4 Accept the one whose faith is weak, without quarreling over disputable matters. 2 One person’s faith allows them to eat anything, but another, whose faith is weak, eats only vegetables. 3 The one who eats everything must not treat with contempt the one who does not, and the one who does not eat everything must not judge the one who does, for God has accepted them. 4 Who are you to judge someone else’s servant? To their own master, servants stand or fall. And they will stand, for the Lord is able to make them stand.
This verse in Romans is really written in the context of those with strong faith and those with weak faith that we do not quarrel over disputable matters. Example given there like one can eat anything, but some whose faith is weak, eats only vegetables. Paul exhorts us to not to treat with contempt the one who does not, and hence so not judge the ones who do eat, because God has accepted them all. It is in that context that Paul says “Who are you to judge someone else’ servant?”
It’s about food, in this context. And Paul asked “who are you to judge one another in their own conscience or faith in terms of eating food according to their faith”. It has nothing to do with disputing someone who has sinned.
Issue of Community Dissension
James 4:11 Brothers and sisters, do not slander one another. Anyone who speaks against a brother or sister or judges them speaks against the law and judges it. When you judge the law, you are not keeping it, but sitting in judgment on it. 12 There is only one Lawgiver and Judge, the one who is able to save and destroy. But you—who are you to judge your neighbor?
This is about community dissension, about slandering one another. Overtly critical spirit, judgmental. This is talking bad on others, quarrelling. Therefore James is talking about do not judge your neighbor because you are judging the law. Believers who judge one another ignore the law, and , in effect, think they are superior to it. So this so called ‘who are you to judge’ is in the context of dissension, very critical spirit, and ignoring the law.
Do not Judge in the sense of Condemning or Judging harshly
Matt 7 “Do not judge, or you too will be judged. 2 For in the same way you judge others, you will be judged, and with the measure you use, it will be measured to you.
3 “Why do you look at the speck of sawdust in your brother’s eye and pay no attention to the plank in your own eye? 4 How can you say to your brother, ‘Let me take the speck out of your eye,’ when all the time there is a plank in your own eye? 5 You hypocrite, first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother’s eye.
The Greek word for ‘judge’ here is Krino meaning to condemn overly judge overly harshly. The main thing here the judgment here refers to overly harshly, condemningly, that’s entirely not what we talk about when we correct or rebuke some wrong doings or sins by our friends or loved ones. To extend blanket view for the word judge therefore completely does not do justice to rebuking and correcting and speaking in truth. Remember, judging critically or even condemning is definitely different from rebuke to speak the truth in love.
We rebuke, not judge someone when they have sinned
Because there is nothing to judge, it is plainly in the scriptures. Scriptures is the judge if you like, we are just following the scriptures.
We are following the word of God, and indeed, if you don’t expose them and speak the truth in love, you are not following the scriptures. Too many times we have heard people say ‘who am I to judge? because I am also a sinner’, when they deal with someone who has sinned and grieved the Lord in a particular area. Of course, we are sinners, but we should be open to rebuke by anyone as well so we repent if we have sinned. But we should not stop rebuke and sweep sins under the carpet. Sin is a sin, we have to confront it, acknowledge it, confess it, and repent. And it is in the job of the community friends to confront and rebuke our friends and loved ones when they sin, in a gentle way, but firm way.
We are Called to Speak the Truth in Love
Ephesians 4 14 Then we will no longer be infants, tossed back and forth by the waves, and blown here and there by every wind of teaching and by the cunning and craftiness of people in their deceitful scheming. 15 Instead, speaking the truth in love, we will grow to become in every respect the mature body of him who is the head, that is, Christ.
If we don’t speak the truth to one another in love, our dear ones will be continually be tossed back and forth by the weaves and winds of the world, the cunning and craftiness of the deceitful scheming of the world. So very much we need to exhort, correct and speak the truth. And don’t drag in the label “Who am I to judge?” Because that’s meant for disputable matters like what food to eat, community dissension, and some very condemning critical sprit. But when you deal with murder, stealing, lying, homosexuality, adultery, etc., this is no disputable matters anymore, this is sin, and we have to speak it and expose the deeds of darkness in love.
Remember if you will have to be perfect before you correct rebuke a sinner friend of yours, there will never be any rebuking and correction.
 Don Carson, NIV Study Bible, Zondervan