Charles Spurgeon Commentary Matthew 17:13

"Then understood the disciples that he spake unto them of John the Baptist." — Matthew 17:13 (ASV)

John had indeed come in the spirit and power of Elias, yet Herod had put him to death, as other wicked men would deal with his Lord and Master whose way he so gloriously prepared.

“Then the disciples understood.” Our Lord’s instructive word opened their understanding. When He teaches, the dullest scholars learn. Now they see that John the Baptist was Elijah come again. He was a stern admonisher of kings and preached repentance to Israel. He had come to restore all things, and so the Messiah had not appeared without being preceded by the true Elias. This was plain enough to them once their Lord had made them understand. Lord, always, not only speak to us, but cause us to comprehend Your word!