His message is repentance--to turn back to God with complete loyalty to him in his kingdom. "Repentance" is a word that is danger for us. Some people misunderstand it as only a call to feel remorse for sin. But for Jesus, it's a loyalty-check: Who or what are you committed to?
This is why he follows his sermon by calling disciples. The response he wants is repentance. But how is repentance seen? It is not seen by disciples feeling sorry for what they did and promising to do better, but by disciples giving up everything, leaving businesses and work and family behind, in order to follow Jesus.
Repentance is following. Follow Jesus.