What does Matthew 18:5-6 mean?

5 And whoso shall receive one such little child in my name receiveth me. 6 But whoso shall offend one of these little ones which believe in me, it were better for him that a millstone were hanged about his neck, and that he were drowned in the depth of the sea. (Matthew 18:5-6 KJV)

William Burkitt’s Commentary

Our Saviour having declared, that the humblest persons should be always highest in his esteem; he next declares how exceeding dear and precious such christians are to him, who resemble little children in humility of heart, and innocency of life: assuring the world, that whatever kindness and respect is shewn to such for his sake, he reckons shewn to himself; and all the disrespect and unkindness which is offered to them, be accounts as done unto himself: so near is the union, and so dear the relation, betwixt Christ and his members, that whatever good or evil is done unto them, he reckons as done unto himself.