Paul is talking about passing judgment on folks in matters that are not central to our faith. He reminds us whose job it is to judge these sorts of matters. He also reminds us that the person we are judging actually belongs to the Lord and what right do we have to pass judgment on that person. So often we can find fault with others, pass judgment on them about some inconsequential matter, but then never deal with the blatant sin in our own life. Let's remember that we will answer to God for what we do every bit as much as someone we are wrongly passing judgment upon.