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If I know that someone is sinning, but don’t tell him, am I sinning?

As Christians, we are called to confront sin and to stand up for truth, in love. Ephesians 4:25 says, “Therefore each of you must put off falsehood and speak truthfully to his neighbor, for we are all members of one body.” Your desire should be that your friend lives a righteous life before God—not in a judgmental way, but because you know that only in that is fullness of life. So, as a good friend, and because you are called to lovingly reproach sin, you should at least be willing to approach your friend.

Now, as to whether you should or not, that is between you and God. You must have the leading of the Holy Spirit for any type of confrontation to be effective. So, you must pray about the situation to see what God wants you to do. While praying, you will also want to ask God to show you the true motives of your heart and to rid your heart of any selfish or judgmental motives.

"Search me O God, and know my heart, try me, and know my thoughts; And see if there be any wicked way in me, and lead me in the way everlasting" (Psalm 139:23-24).