“Whoever receives a child, receives Jesus” refers to showing basic hospitality towards those who are in need. Jesus says something similar in Matthew 10:40-2 where the “little ones” are the disciples and 25:35-40 where the “least of these” are given hospitality by the “sheep.” In this saying in Matthew 18:5-6 Jesus is warning his disciples that causing one of these “little ones” to sin is a very grave offense.
To “cause someone to sin” is a single word (σκανδαλίζω) often associated with being offended, or causing someone to stumble (the weaker and stronger brothers in Romans 14-15). The verb refers to putting something in the way of another person to cause them to stumble (think of a football player to throws himself at the feet of another player to cause him to trip). In Matthew 11:6 Jesus used the word to describe those who doubt who he really is, they are “offended” on account of Jesus.
The verb is also used in the parable of the sower for some of the seed that falls on the rocky ground. Because the plant had no roots, it withers up when persecution comes. I suggest this is the kind of person who hears the gospel and seems to accept it, but something happens which causes them to fall away before they have produced fruit. If the Parable of the Sower is a valid guide, then this is more than causing a child to sin. Someone is preventing a child from hearing the gospel and coming to a saving faith.
Jesus says it is better for the one who prevents a child coming to saving faith to never have existed. In the context, this refers to the disciples who were preventing children from coming to Jesus! “There are hardly suitable translations for the verse’s keywords, the verb σκανδαλίζειν and the substantive σκάνδαλον” (Luz, Matthew 8-20, 432).
This person would be better off to have a heavy millstone hung around their neck and tossed into the sea! Even small millstones used by women to bring wheat were very heavy, but this is the kind of millstone turned by a donkey. An “ass’s millstone” (μύλος ὀνικὸς) refers to the upper stone of a stone mill, heavy enough that it must be turned by a donkey, horse, or slave. It would result in a quick and inescapable death. “Drowning was apparently used by the Romans as a form of capital punishment and was presumably known as such in Palestine” (Hooker, Mark, 232). Keener cites Livy 1.51.9, a person was stuffed in a crate of stones and tossed into the sea (Keener, Matthew, 449).
This warning is aimed at the one who prevents the little one from coming to saving faith, a common theme in Matthew (11:6; 13:57; 15:12; 26:31, 33) and apostasy (13:21; 24:10). That he is thinking here also of leading people into apostasy is obvious.
Who are these little ones, and who are leading them astray? In the context of Matthew’s Gospel, Jesus may have the child from 18:2 in mind (assuming he is still standing there). However, as I said above, Jesus calls his disciples “little ones” several times in Matthew (10:42, for example). This is a warning to those who are leading the disciples after the resurrection to be very careful about how they present Jesus’s teaching to the growing church.
Does this have an application to Matthew’s original audience? Although it is far from certain, a common suggestion is that Matthew wrote his Gospel in the context of the church a Syrian Antioch. If this is the case, the “stumbling blocks” may be individuals in those churches who are teaching something other than what Jesus taught his disciples. This may be variation in theology or practice, and at that point in history it may not be a major heresy. But anything which leads the true disciples of Jesus into sin is extremely dangerous.
This ought to be convicting to anyone involved in ministry. It is obvious that what you teach and preach can drive people away from the Gospel, but how you interact with people and how you behave is equally offensive. I am not talking here about dropping key doctrines of the faith because they are strange to the modern listener. Something like, “since people do not understand the Trinity anymore I am going to drop Trinity from the church doctrinal statement.” Pick any given study of why people are leaving the church, the reason given is hypocrisy of leaders of the church. Although this might sometimes be a perception rather than a reality, there are plenty of examples of pastors failing morally while demanding their congregations avoid those very sins. There are several recent examples of big name pastors who support politicians who are objectively immoral to please their constituency.
If this is the case, we are going to need a lot more millstones.