A promise, and a warning! The Gospel reading is the familiar story about the Prodigal Son. We know the story of the son who ran off and spent away his inheritance, and then came home to forgiveness.
But the Epistle reminds us of the people led by Moses. They were given food and drink by God, but nevertheless, some transgressed and faced some nasty consequences.
The point, then, is that once you have been saved, it remains necessary to maintain your salvation, and get forgiveness when you need it!