An excerpt from "Ligonier.org" entitled, "Causing Others To Sin" states: "We can commit no greater evil against another believer than causing them to sin. After all, we impede their growth into Christlikeness and negatively affect their heavenly reward when we lead other Christians astray. So, it follows that treating other Christians properly entails doing what we can to avoid causing them to stumble. The end that awaits those who cause others to sin is horrible indeed. It is so awful that it is better for the person who leads others into transgression to have a "great millstone" hung around his neck and to be hurled in the sea. In the ancient world, a millstone was a stone placed on top of another stone and turned in order to crush grain into flour. The millstone was so heavy that it took several animals to turn it. To be thrown into the sea with a millstone around one's neck would be certain death. The terror of this image would have been particularly real to Jesus' original first-century Jewish hearers, for ancient Jews feared the sea, making death by drowning a particularly horrific end."
This article may be viewed at: Causing Others to Sin/ligonier.org
Reference: 1 Corinthians 8:12