Why is the word "if" important in Matthew 11:14?
The word "if" in Matthew 11:14 makes the identification of John the Baptist as Elijah conditional upon Israel's receiving it. Jesus was speaking to Jews, the nation of Israel, so the fulfillment of Malachi's Elijah prophecy depended on their acceptance, not an automatic, unconditional event.
Les points out that "if" is a little word easy to gloss over, yet it is loaded. In Matthew 11:14 Jesus says, "And if you will receive it, this is Elias." The context reaches back to Malachi 4:5-6, where God promises to send Elijah before the great and dreadful day of the Lord, and Elijah will turn the hearts of fathers and children. Jesus is addressing Jews, the nation of Israel. So the "if" shows that receiving John as the promised Elijah was conditional on their response. The surrounding verses note that John the Baptist was the greatest born of women, that the least in the kingdom is greater, that the kingdom suffered violence, and that the law and prophets prophesied until John. The "if" prevents assuming an unconditional fulfillment; it depended on Israel's acceptance.
And if ye will receive it, this is Elias, which was for to come.