Contradiction with Malachi 4:5
While Mark 9:13 suggests that Elijah has already come, Malachi 4:5 foretells Elijah will come before the "great and dreadful day of the Lord," implying a future event.
Malachi 4:5: Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
Contradiction with Matthew 11:14
In Mark 9:13, Elijah's coming is implied as having been fulfilled, whereas Matthew 11:14 indicates John the Baptist, if accepted, is Elijah, creating ambiguity about the role's fulfillment.
Matthew 11:14: And if ye will receive [it], this is Elias, which was for to come.
Contradiction with John 1:21
In Mark 9:13, the fulfillment of Elijah's coming is suggested while John 1:21 shows John the Baptist denying he is Elijah, contradicting the implication in Mark 9:13 about Elijah's presence.
John 1:21: And they asked him, What then? Art thou Elias? And he saith, I am not. Art thou that prophet? And he answered, No. [that prophet: or, a prophet?]
Contradiction with Matthew 17:11-12
Mark 9:13 infers the past arrival of Elijah, while Matthew 17:11-12 suggests Elijah's coming as a corrective for Israel, indicating an expectation of future events.
Matthew 17:11-12: And Jesus answered and said unto them, Elias truly shall first come, and restore all things.