Jesus replied, “To be sure, Elijah comes and will restore all things. But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands.”
They knew John the Baptist had been killed, and now Jesus is indicating the same will happen to Him. When they understood, they must have been very sad. Then, He confirms it – verses 22 and 23 – “When they came together in Galilee, he said to them, “the Son of Man is going to be betrayed into the hands of men. They will kill him, and on the third day he will be raised to life.” He is going to die.
They had just been on the mountain top in the midst of the transfiguration (vs 1-9), and suddenly, they are deep in the valley of sorrow. Let us take note: HE IS WITH THEM IN BOTH PLACES!