Malachi 4:5 - Wesley's Explanatory Notes

Behold I will send — Though the spirit of prophecy cease for four hundred years, yet at the expiring of those years, you shall have one sent, as great as Elijah. Elijah — Namely John the Baptist, who came in the spirit and power of Elijah, Luke 1:17, and therefore bears his name. Before — That is, immediately before; so he was born six months before Christ, and began his preaching a few years before Christ began to exercise his publick office. The great and dreadful day of the Lord — This literally refers to the times of vengeance upon the Jews, from the death of Christ to the final desolation of the city and temple, and by accommodation, to the end of the world.