Matthew 17:23

“And they will kill him, and the third day he will be raised up.” And they were very upset.'

And He then makes clear what will follow. ‘They will kill Him.' He is in no doubt about what His fate will be. Man is to be allowed to do His worst. But the last word will be with His Father. On the third day He will be raised up. Jesus has total faith in His Father. For the rising up on the third day see on Matthew 16:21. ‘The third day' might simply signify ‘within a short time of less than a week', being in contrast with ‘seven days' (compare the use of ‘three days' and ‘seven days' in Genesis).

The concentration of the disciples, in so far as they understand it at all, is on His words about death, and they are therefore very upset at this talk of death. They still cannot really bring themselves to believe it.