See the notes at Mark 8:31.

But first must he suffer many things - As the cup of the iniquity of this people shall not be full till they have finally rejected and crucified the Lord of life and glory, so this desolation cannot take place till after my death.

But first must he suffer many things,.... By cruel mockings, spitting, buffeting, scourging, and, at last, death itself; all which must be, and were before his day came, or he entered into his glory, or came in it:
and be rejected of this generation; as the Messiah, and be treated with the utmost scorn and contempt, and in the most base and ignominious manner: being put to the death of the cross, and hanged upon the accursed tree: all which were necessary, "must" be; on account of the purposes and decrees of God; the covenant engagements of Christ; the predictions of the prophets of the Old Testament, and his own; and the salvation of his people.

But first . . . suffer, &c.--This shows that the more immediate reference of Luke 17:23 is to an event soon to follow the death of Christ. It was designed to withdraw the attention of "His disciples" from the glare in which His foregoing words had invested the approaching establishment of His kingdom.

See notes on Matthew 16:21 and Matthew 24:37-39. Compare Colossians 3:3-4; 2-Thessalonians 1:7; 1-Peter 1:7.