Adam Clarke on Mark 9:32

But they understood not - This whole verse is wanting in two MSS., in the first edition of Erasmus, and in that of Aldus. Mill approves of the omission. It does not appear likely, from Matthew's account, that three of the disciples, Peter, James, and John, could be ignorant of the reasons of Christ's death and resurrection, after the transfiguration; on the contrary, from the circumstances there related, it is very probable that from that time they must have had at least a general understanding of this important subject; but the other nine might have been ignorant of this matter, who were not present at the transfiguration; probably it is of these that the evangelist speaks here. See the observations on the transfiguration, Mat 17:9 (note), etc., and Mat 18:1 (note).