"In the spirit and power of Elijah" indicates John resembled Elijah in doing a similar work of revealing the true God through a ministry devoted to preaching repentance and the certainty of things contained in the Scriptures regarding Christ. Perhaps it also includes working with a similar zeal, though he accomplished his function without miracles (John 10:41). Obviously, God does not measure a man's greatness by the miracles he does.
On two separate occasions, in Matthew 11:13-15 and again in Matthew 17:10-13, Jesus says John is the Elijah to come. Notice first Matthew 11:13-15: "For all the prophets and the law prophesied until John. And if you are willing to receive it, he is Elijah who is to come. He who has ears to hear, let him hear!" Let him who has ears, listen! Jesus wants His audience to pay the utmost attention. To what? To the fact that John is the Elijah to come! He had fulfilled Malachi 4:5-6.
Notice, too, Jesus' introductory comment in verse 14, "And if you are willing to receive it. . . ." This strongly suggests that He was about to say something different than what His listeners expected. They supposed Elijah would appear in person! This explains why, when John was asked by the delegation from Jerusalem whether he was Elijah, he replied, "I am not" (John 1:21). Though he was Elijah in spirit and power, he was not the literal Elijah they were expecting. The Jews of Jesus' day were just as wrong about Elijah as are many today who are looking for another Elijah to appear before Jesus' second coming. Yet, Jesus gives no indication that anyone will follow John in that office.