If ye will receive it - This is a mode of speaking implying that the doctrine which he was about to state was different from their common views; that he was about to state something which varied from the common expectation, and which therefore they might be disposed to reject.
This is Elias ... - That is, “Elijah.” Elias is the “Greek” mode of writing the Hebrew word “Elijah.” An account of him is found in the first and second books of Kings. He was a distinguished prophet, and was taken up to heaven in a chariot of fire, 2 Kings 2:11. The prophet Malachi Malachi 4:5-6 predicted that “Elijah” would be sent before the coming of the Messiah to prepare the way for him. By this was evidently meant, not that he should appear “in person,” but that one should appear with a striking resemblance to him; or, as Luke Luke 1:17 expresses it, “in the spirit and power of Elijah.” But the Jews understood it differently. They supposed that Elijah would appear in person. They also supposed that Jeremiah and some other of the prophets would appear also to usher in the promised Messiah and to grace his advent. See Matthew 16:14; Matthew 17:10; John 1:21. This prevalent belief was the reason why he used the words “if ye will receive it,” implying that the affirmation that “John” was the promised Elijah was a doctrine contrary to their expectation.