Jesus Christ said in His Sermon on the Mount: "Judge not, that you be not judged. For with what judgment you judge, you will be judged; and with the measure you use, it will be measured back to you" (Matthew 7:1-2). Therefore, many assume that it is wrong to pass judgment of any sort, and that all people should be allowed to do whatever they feel is right.
Strong's Concordance notes that the word translated "judge" in Matthew 7:1 can also mean "condemn." What Christ was saying was that, as we are incapable of seeing a person's heart or knowing his or her relationship with God, we are not to take the place of God in making judgments about someone's motives or eternal salvation. And we should be humble, knowing our own weaknesses and sins.
However, in other sections of the Bible, Christ says very clearly, "Out of the abundance of the heart the mouth speaks" (Matthew 12:34) and "a tree is known by its fruits" (Matthew 12:33). While we cannot see hearts, we can see—and judge—actions. There are times God calls on us to discern others' actions because of their effects on us. For example, the Bible tells us to avoid angry people and those whose sins may rub off on us (Proverbs 22:24; 1 Corinthians 5:11-12). Hebrews 5:14 tells us that the spiritually mature are "those who by reason of use have their senses exercised to discern both good and evil."
We are not to judge where a person stands with God, because we are simply incapable of doing so. But we must be able to "discern both good and evil" in the actions of ourselves as well as others in order to grow in wisdom and avoid falling into problems ourselves.
For an additional study tool, please read our booklet The New Covenant: Does it Abolish God's Law?