Matthew 18:6
but whoever causes one of these little ones who believe in me to stumble, it would be better for him that a huge millstone should be hung around his neck, and that he should be sunk in the depths of the sea.
WEB
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.
KJV
What Matthew 18:6 means
Jesus warns that harming or corrupting one of the vulnerable “little ones” who trust him is so serious that drowning in the sea would be a lighter fate than the judgment it invites.
Jesus says this in the middle of an argument. His disciples had just asked him who would be the greatest in God's kingdom. Instead of answering with a name, he called a small child over and stood the child in front of them. In his world a child had no rank, no power, and no say. Jesus told the men chasing greatness that the kingdom belongs to people like this, and then he turned to what happens to anyone who harms them.
“These little ones” means more than children. It covers everyone who is small and newly trusting, the people in a community with the least power to defend themselves. To “cause one to stumble” is stronger than hurting their feelings. The word behind it is the root of our word “scandal,” and it means to trip someone into sin or to wreck the trust that holds their faith together.
The picture Jesus reaches for is deliberately extreme. A millstone was the heavy upper stone of a mill, the large kind turned by a donkey rather than by hand. To be sunk in the open sea with that stone around your neck was ruin with no bottom and no return. He is not writing a rule about drowning. He is measuring something: how seriously God takes harm done to the defenseless. The lines right after say it from the other side, comparing God to a shepherd who leaves ninety-nine sheep to chase the one that wandered off, because it is not his will that a single little one be lost.