17:22 And while they abode in Galilee, Jesus said unto them, The Son of man shall be betrayed into the hands of men:

17:23 And they shall kill him, and the third day he shall be raised again. And they were exceeding sorry.

Jesus declared his death, and resurrection. The disciples were sorry because they did not understand what was foretold in the law and the prophets, or by Jesus himself, about his death, and resurrection. Luke 18:33 John 18:32 Psalms 22:11-22 Psalm 16:10

They did not understand that his death was necessary to accomplish the new testament, or that his death, and resurrection, brought salvation by grace. Hebrews 9:16 1 Corinthians 15:3-4 Romans 5:8 Romans 4:25

Luke 9:1 Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases.

9:2 And he sent them to preach the kingdom of God, and to heal the sick.

The twelve disciples were sent to preach the kingdom of God; the prophetic kingdom on earth, promised to Israel.

9:6 And they departed, and went through the towns, preaching the gospel, and healing every where.

The twelve were not preaching the same gospel as Paul, but rather, the gospel of the kingdom. They did not understand that the cross, the death, and resurrection of Jesus, would be the gospel, revealed to Paul.