John Gill Commentary Mark 9:10

"And they kept the saying, questioning among themselves what the rising again from the dead should mean." — Mark 9:10 (ASV)

And they kept that saying with themselves
"They retained it in their own mind", as the Persic version renders it; "they kept [it] close", as Luke says, (Luke 9:36) , among themselves, and acquainted no man with it: and which refers either to the whole of Christ's charge, relating to the vision on the mount; or else only to what he said about his resurrection from the dead; and which they took notice of particularly, and laid hold upon, as the word will bear to be rendered; and so the Ethiopic version does render it, "and they observed his saying"; what he last said concerning the son of man's rising from the dead;

questioning with one other what the rising from the dead should
mean :
they inquired, disputed, and reasoned with one another, what should be the meaning of such an expression: not that they were ignorant of the general resurrection of the dead; for this was the hope of Israel, and the general sense of the Jewish nation: but they did not know what he meant by his particular rising from the dead: whether he meant it in a literal sense, which supposed his death; and that though he had lately told them of, they knew not how to reconcile to the notions they had of a long and flourishing temporal kingdom of the Messiah; or whether he meant a and interest, in such manner as they expected.