Mark 9:42
And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck, and he were cast into the sea. And whosoever shall offend one of these little ones that believe in me, it is better for him that a millstone were hanged about his neck, and he were cast into the sea.
The warning belongs to the lesson in which Jesus took a child to rebuke the rivalry of the disciples over who should be greatest. In that setting He taught that true greatness in God's sight is given to those who are humble and serve, and that to receive a child in Christ's name is to receive Christ Himself and the One who sent Him. The little ones that believe in Him are His disciples and believers who serve Him, however humble they may be.
To offend such a one is to injure or hinder him, to cause him to stumble and suffer spiritual harm, and to mislead him and turn him away from Christ. This offending includes misleading a believer by willful deception or by a wrong example, and making him an object of slander or ridicule. It also includes misrepresenting Christ, and by harshness, coldness, or indifference driving into the enemy's path those whom Christ is drawing. It even includes forbidding another person to labor for Christ merely because he does not follow one's own plans.
Such conduct is a grave sin that insults Jesus Himself, for every wrong done to His followers is marked by Him and brings severe guilt upon the offender. Sudden destruction by drowning with a millstone hung about the neck would be better than causing the loss of a soul. Because Christ came to save the lost and every soul was bought at infinite cost, His followers must instead care for the souls of others. They must help those who are tempted by drawing them to Christ for the grace and help they need, and must work for one another, pray for one another, and encourage one another.