There are three sections in the reading today. Two are appealing, one is confronting.
In the first section, the apostle John of all people is complaining to Jesus that people who "are not one of us" are casting out devils in the name of Jesus. Jesus says, "anyone who is not against us is for us." The liturgy clearly associates John's opponents with Eldad and Medad in the First Reading. Moses and Jesus both express the same thought. "If only the whole people of the Lord were prophets, and the Lord gave his Spirit to them all." "No one who works a miracle in my name is likely to speak evil of me."
To act in the "name of Jesus" is to act as Jesus. Note "casting out in the name of Jesus" and "a cup of water to those who bear the name of Christ." (NRSV) This verse speaks to Christian values.
We are lucky as Catholics to have the guidance of the Church in interpreting some passages. The Church has never understood passages about self-mutilation literally. There are some famous examples of the Church condemning such practices. (Google: Origen, Abelard). I don't thing we can read this passage without remembering ancient as well as recent scandals in the Church.
The confronting teaching of Jesus is to avoid scandal (obstacle = obex = scandal). Jesus uses Middle-Eastern hyperbole and exaggeration to make a point. And what a point it is. "It would be better to be thrown into the sea..."
There are three lessons. Those who are "not one of us," the reward for a cup of water, and the penalty for giving scandal. Could easily be three separate sermons.