Sons of the kingdom will be cast out.The phrase
sons of the kingdom was a term the Jews used of themselves, based on their lineage from Abraham (Matt. 3:9; Luke 3:8). Jesus is saying that believing Gentiles will enter the kingdom, but unbelieving Jews will be cast out.
Outer darkness … weeping and gnashing of teeth. These phrases were well known to the Jews of Jesus’ time, being used by the rabbis for the torments of Hell. See Luke 13:27-30, where Jesus is speaking about unbelievers.