Elijah indeed will come first. We have stated elsewhere the origin of that error which prevailed among the Jews. As John the Baptist was to resemble Elijah by restoring the fallen condition of the Church, the prophet Malachi (4:5,6) had even given to him the name of Elijah; and this had been rashly interpreted by the scribes, as if Elijah the Tishbite (1 Kings 17:1) were to return a second time to the world. Christ now declares that every thing which Malachi uttered was true, but that his prediction had been misunderstood and distorted from its true meaning. "The promise," says he, "that Eliah would come was true, and has been already fulfilled; but the scribes have already rejected Elijah, whose name they idly and falsely plead in opposing me." And will restore all things. This does not mean that John the Baptist restored them perfectly, but that he conveyed and handed them over to Christ, who would complete the work which he had begun. Now as the scribes had shamefully rejected John, Christ reminds his disciples that the impostures of such men ought not to give them uneasiness, and that it ought not to be reckoned strange, if, after having rejected the servant, they should, with equal disdain, reject his Master. And that no one might be distressed by a proceeding so strange, our Lord mentions that the Scripture contained predictions of both events, that the Redeemer of the world, and Elijah his forerunner, would be rejected by false and wicked teachers.

Elias truly shall first come, and restore all things - He did not mean by this that Elijah was yet to come, for he tells them immediately Matthew 17:12 that he had come; but he meant to affirm that it was a true doctrine which the scribes taught, that Elijah would appear before the coming of the Messiah. To "restore" means to put into the former situation. See Matthew 12:13. Hence, it means to heal, to correct, to put in proper order. Here it means that Elijah would put things in a proper state; he would be the instrument of reforming the people, or of restoring them, in some measure, to proper notions about the Messiah and preparing them for his coming. Before the coming of John their views were erroneous, their expectations were worldly, and their conduct were exceedingly depraved. He corrected many of their notions about the Messiah (see Matt. 3), and he was the instrument of an extensive reformation, and thus restored them, in some degree, to correct views of their own system and of the Messiah, and to a preparation for his advent.

Elias - shall first come, and restore all things - Or will reform, αποκαταστησει; this word our Lord quotes from the Septuagint; who render the Hebrew והשוב לב אבות על בנים vehesheb leb aboth al banim, he will cause the heart of the fathers to turn to the children, by, ος αποκαταστησει καρδιαν πατρος προς υιον, who will convert, or restore, the heart of the father to the son. We are not therefore to understand the version of the Septuagint quoted by our Lord in any other sense than the Hebrew will allow. No fanciful restoration of all men, devils and damned spirits, is spoken of as either being done, or begun, by the ministry of John; but merely that he should preach a doctrine tending to universal reformation of manners, and should be greatly successful: see Matthew 3:1-7, and especially Luke 3:3-15, where we find that a general reformation had taken place,
1. among the common people;
2. among the tax-gatherers; and
3. among the soldiers.
And as John announced the coming Christ, who was to baptize with the Holy Ghost, i.e. to enlighten, change, and purify the heart, that the reform might be complete, both outward and inward, he may be said, in the strictest sense of the word, to have fulfilled the prophecy: and that he was the Elijah mentioned by Malachi, the words of Gabriel to the virgin Mary prove; Luke 1:17. And he (John) shall go before him (Christ) in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, etc.; and that his ministry was powerfully effectual for this purpose, we have already seen.

And Jesus answered and said unto them,.... By way of concession,
Elias truly shall first come: this is indeed a tenet of the Scribes, and it is also certain, that there is a prophecy in Malachi 4:5 of the coming of Elias; of one that goes under that name, not of Elias the Tishbite, in person, but of one that was to come in his power and spirit,
and restore all things. The Syriac and Persic versions render it, "shall perfect, or complete all things", that are prophesied of him; and shall put a period to the law and the prophets, and close the Mosaic economy, and direct persons to Christ; in whom are the perfection of the law, and the fulfilling of the prophets. The Arabic version reads it, "he shall teach you all things"; the whole of the Gospel being to be reduced to these two heads, repentance towards God, and faith in Christ; both which were taught by the true Elias: but the truest sense of the phrase is to be learned out of Malachi 4:6. "He shall restore, he shall turn all things, the heart of the fathers to the children, and the heart of the children to their fathers"; and as this is explained in Luke 1:17 "he shall turn the disobedient to the wisdom of the just, and make ready a people prepared for the Lord": which is other, and better sort of work, than what the Jews assign to their Elias, whom they expect, and whom they make to be a restorer of all things, in their way: they often speak of his purifying (q) of things, or pronouncing things pure, that were defiled; and among others, that he will purify bastards, and "restore them" to the congregation of the Lord (r). Though Maimonides (s) denies, that when he comes he will pronounce defiled that which is pure, or pronounce pure, that which is defiled. They pretend (t), that he is now employed, and very busy, in writing everything that is done in every age; so that when he comes, he will be able to give an account of everything: and nothing is more common with them, than to say concerning any matter, that there is any doubt or difficulty about it (u), , "let it be left till Elias comes".
(q) T. Bab. Chagiga, fol. 25. 1. & Becorot, fol. 33. 2. & 34. 1. (r) Kimchi in Zech. ix. 6. (s) Hilchot Melacim, c. 12. sect. 2. Vid. Misn. Ediot, c. 8. sect. 7. & Maimon & Bartenora in ib. (t) Seder Olam Rabba, p 46. (u) Misn. Bava Metzia, c. 1. sect. 8. T. Bab. Bava Metzia, fol. 3. 1. & 37. 1. & Bava Bathra, fol. 94. 1.

Elias (Elijah) is come already. John the Baptist, who came in the spirit and power of Elijah (Mark 1:2-8; Luke 1:17).
Have done unto him whatsoever they listed. List and lust were originally one word, meaning to desire or wish. The account of his martyrdom is given in Matthew 14:6-12; Mark 6:21-29. See notes there.
The Son of man suffer of them. Henceforth he keeps the lesson of his suffering constantly before their minds. After all, his disciples were not prepared for it when the hour came.

Regulate all things - In order to the coming of Christ.