In a world of sinners, how do born-again believers stand for Christ? Jesus tells us. Go into all the world and preach the gospel. It is the gospel that saves. Judging the world does not save. However, sometimes that line gets blurred when the church begins to think we don't judge anyone--ever.
In New Testament doctrine, Christians must judge other Christians who actively call themselves Christians and who present themselves as leaders. This is an important distinction because new Christians don't need rebuke, but they do need reproof, which is a gentler correction. Yet, Christians who should know better need rebuke. Otherwise, Christianity will pay for it, generations will pay for it, and the one who allowed it to begin and withheld their power to remove it will answer for it.
Though this is a hard choice for some, it is better to fear God and do what He says than to be a stubborn castaway.
For what have I to do to judge them also that are without? do not ye judge them that are within? But them that are without God judgeth. Therefore, put away from among yourselves that wicked person. (1 Corinthians 5:12-13 KJV)