Mar 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.
The Preceding Context:
Mar 9:38 And John answered him, saying, Master, we saw one casting out devils in thy name, and he followeth not us: and we forbad him, because he followeth not us.
Mar 9:39 But Jesus said, Forbid him not: for there is no man which shall do a miracle in my name, that can lightly speak evil of me.
Mar 9:40 For he that is not against us is on our part.
Mar 9:41 For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ, verily I say unto you, he shall not lose his reward.
The preceding context concludes with a promise that even just the gift of a cup of water to drink shall be rewarded accordingly.
Then follows a contrasting warning that whoever may offend “one of these little ones that believe in me,” it would be “better for him that a millstone were hanged about his neck, and he were cast into the sea.”
More insight can be gained by a study of what the rest of the Bible teaches about “offend.”
Here are the cross references for your study:
offend. or, shall have caused to stumble. Mar 14:18, Isa 9:16, +Mat 17:27; *Mat 18:6; *Mat 18:10, *Luk 17:1; *Luk 17:2, Rom 14:13; Rom 14:15; Rom 14:21; Rom 16:17, 1Co 8:7, 8, 9, 10, 11, 12, 13; 1Co 10:32, 33, 2Co 6:3, Php 1:10, 1Ti 5:14, 15, 2Pe 2:2.