Peter,James and John did not understand Mark 9:9

9 And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead. 10 And they kept that saying with themselves, questioning one with another what the rising from the dead should mean. 11 And they asked him, saying, Why say the scribes that Elias must first come? 12 And he answered and told them, Elias verily cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought. 13 But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.