What does Matthew 18:6 mean?
Jesus has been describing the child-like faith and humility required to enter the kingdom of heaven (Matthew 18:1–5). Now He seems to be describing those who believe in Him, including adults, as "little ones."He warns that a harsh judgment will come on those who cause one of the little ones who believe in Him to sin or to stumble. This would include falling away from faith in and commitment to Christ. It would be better for that person to have a large millstone tied around his neck and to be drowned in the deep part of the sea. Drowning was a form of execution used in the ancient world, but rarely in Israel. Jesus' harsh description of death by drowning includes being strapped to the enormous and heavy millstone pulled by a donkey as it crushes the grain. This would assure no hope of escape.
Jesus has regularly warned about a judgment to come when He returns to earth with His angels, where people will be repaid for what they have done, including their sin (Matthew 16:27). This is the first time, though, that Jesus has mentioned a judgment for those who cause others to sin, including leading those who believe in Him into sin.
Jesus' warning here reveals that believers in Jesus are not perfect and can sin and that the consequences are serious both for them and for those who lead them into that sin. Peter, for instance, will commit the grave sin of denying faith in and relationship to Jesus. This will bring him great sorrow, but he will be restored and go on to be used by God to accomplish powerful things for Christ.