That line is in our hearts. When we habitually or intentionally look for flaws in others, we are on the wrong side of the line. We’re judging, then, in the way Jesus condemns. When we see the sins of others and seek to help them by prayer, forgiveness or service, then we are on the right side of the line. We’re judging, then, correctly. The line of demarcation is clear if our hearts are pure.
Paul detailed the line in his treatise on love. “Love is patient, love is kind. It does not envy, it does not boast, it is not proud. It is not rude, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. Love does not delight in evil but rejoices with the truth. It always protects, always trusts, always hopes, always perseveres” (I Corinthians 11:4-7). In other words, no one who loves another can at the same time judge them incorrectly. The line of demarcation is between love and the lack of love in our hearts.