But they didn't understand what this meant. Its meaning was hidden from them so they didn't realize its implications, and they were afraid to ask him about it.
At the time, Jesus' disciples did not understand what these things meant. It was only later when he was glorified that they realized what had happened had been prophesied and applied to him.
The crowd responded, “The Law tells us that the Messiah will live forever, so how can you say the Son of man must be ‘lifted up’? Who is this ‘Son of man’?”