The 2002 LDS booklet titled Preparing to Enter the Holy Temple states:
If we would understand both the history and the doctrine of temple work we must understand what the sealing power is. We must envision, at least to a degree, why the keys of authority to employ the sealing power are crucial.
Nearly nine hundred years before Christ, the prophet Elijah appeared in the court of the king of Israel. He carried with him a sacred authority: the power to seal.
Elijah worked out his ministry, ordained and anointed Elisha to succeed him, and then—and this is important—he did not die. Like Moses before him, he was translated.
After that, his name appears only once in the Old Testament, in the next to the last verse of the last chapter of the Old Testament. It is here that Malachi prophesies that Elijah would return and that he would “turn the heart of the fathers to the children, and the heart of the children to their fathers,” lest the whole earth be smitten with a curse. (See Malachi 4:5–6.)
Mormon leaders are free to interpret passages any way they want, but they are also free to be wrong. This passage has nothing to do with “sealing powers” that can somehow be done in the temple. This is something the Mormon leaders have made up.
Malachi 4:5-6 says,
See, I will send the prophet Elijah to you before that great and dreadful day of the Lord comes. He will turn the hearts of the parents to their children, and the hearts of the children to their parents; or else I will come and strike the land with total destruction.
This is the same person as “My messenger” as refered to in Malachi 3:1, which says “will prepare the way before me.” According to the New Testament, this person is John the Baptist (Matthew 11:14; 17:10; Mark 9:11-13; Luke 1:17). Jesus said in Matthew 11:
11 Truly I tell you, among those born of women there has not risen anyone greater than John the Baptist; yet whoever is least in the kingdom of heaven is greater than he. 12 From the days of John the Baptist until now, the kingdom of heaven has been subjected to violence, and violent people have been raiding it. 13 For all the Prophets and the Law prophesied until John.14 And if you are willing to accept it, he is the Elijah who was to come. 15 Whoever has ears, let them hear.
In case the reader didn’t understand the meaning behind this teaching, Jesus provided more information in Matthew 17:10-13. It says,
10 The disciples asked him, “Why then do the teachers of the law say that Elijah must come first?” 11 Jesus replied, “To be sure, Elijah comes and will restore all things. 12 But I tell you, Elijah has already come, and they did not recognize him, but have done to him everything they wished. In the same way the Son of Man is going to suffer at their hands.” 13 Then the disciples understood that he was talking to them about John the Baptist.
To pull a verse out of the Book of Mormon merely begs the question: What evidence do we have that a man/angel named “Moroni” ever existed? We are left with having to put complete trust and faith in Joseph Smith. Mangling an Old Testament passage and then adding a unique LDS scripture is not sufficient support for a Christian.
Indeed, the interpretation that Smith comes up with, seemingly out of the blue, does not hold to what Jesus explained in Matthew as well as Mark (9:11-13), Luke (1:17), and John (1:21).