Albert Barnes on Luke 9:45

It was hid from them. They had imbibed the common notions of the Jews that he was to be a prince and a conqueror, to deliver the nation. They could not understand how that could be, if he was soon to be delivered into the hands of his enemies to die. In this way it was hid from them--not by God, but by their previous false belief. And from this we may learn that the plainest truths of the Bible are unintelligible to many because they have embraced some belief or opinion before which is erroneous, and which they are unwilling to abandon. The proper way of reading the Bible is to lay aside all previous opinions and submit entirely to God. The apostles should have supposed that their previous notions of the Messiah were wrong, and should have renounced them. They should have believed that what Jesus then said was consistent with his being the Christ. So we should believe that all that God says is consistent with truth, and should forsake all other opinions.

(o) "But they understood not" Mar 9:32, Lk 2:50, 18:34