Q. "From where do Jehovah's Witnesses derive the doctrine that Jesus was and now is Michael the Archangel?"
A. The spirit creature called Michael is not mentioned often in the Bible. However, when he is referred to, he is in action. In the book of Daniel, Michael is battling wicked angels; in the letter of Jude, he is disputing with Satan; and in Revelation, he is waging war with the Devil and his demons. By defending Jehovah’s rulership and fighting God’s enemies, Michael lives up to the meaning of his name—“Who Is Like God?” But who is Michael?
At times, individuals are known by more than one name. For example, the patriarch Jacob is also known as Israel, and the apostle Peter, as Simon (Genesis 49:1, 2; Matthew 10:2).
Likewise, the Bible indicates that Michael is another name for Jesus Christ, both before and after his life on earth.
Let us consider Scriptural reasons for drawing that conclusion.
Archangel. God’s Word refers to Michael as “the archangel.” (Jude 9) This term means “chief angel.” Notice that Michael is called the archangel. This suggests that there is only one such angel (the head and chief of ALL the heavenly army of angelic beings). In fact, the term “archangel” occurs in the Bible only in the singular, never in the plural. Moreover, Jesus is linked with the office of archangel. Regarding the resurrected Lord Jesus Christ, 1 Thessalonians 4:16 states: “The Lord himself will descend from heaven with a commanding call, with an archangel’s voice.” Thus the voice of Jesus is described as being that of an archangel. This scripture therefore suggests that Jesus himself is in fact the archangel Michael.
Army Leader. The Bible states that “Michael and his angels battled with the dragon . . . and its angels.” (Revelation 12:7) Thus, Michael is the Leader of an army of faithful angels. Revelation also describes Jesus as the Leader of an army of faithful angels (Revelation 19:14-16). And the apostle Paul specifically mentions “the Lord Jesus” and “his powerful angels.” at 2 Thessalonians 1:7.
So the Bible speaks of both Michael and “his angels” and Jesus and “his angels.” (Matthew 13:41; 16:27; 24:31; 1 Peter 3:22) Since God’s Word absolutely nowhere indicates that there are two armies of faithful angels in heaven—one headed by Michael and one headed by Jesus—it is logical to conclude that Michael is none other than Jesus Christ in his heavenly role.
Again, Michael is the only holy angel other than Gabriel *named in the Bible*, and the only one called “archangel.” (Jude 9) The first occurrence of the name "Michael" is found in the tenth chapter of Daniel, where Michael is described as “one of the foremost princes”; he came to the aid of a lesser angel who was opposed by “the prince of the royal realm of Persia” (a demon). Michael was called “the prince of [Daniel’s] people,” “the great prince who is standing in behalf of the sons of [Daniel’s] people.” (Daniel 10:13, 20, 21; 12:1) This points to Michael as the angel who led the Israelites through the wilderness. (Exodus 23:20, 21, 23; 32:34; 33:2)
Lending support to this conclusion is the fact that “Michael the archangel had a difference with the Devil and was disputing about Moses’ body.”—Jude 9.
Scriptural evidence indicates that the name Michael applied to God’s Son before he left heaven to become Jesus Christ and also after his return to heaven. As already stated, Michael is the only one said to be “the archangel,” meaning “chief angel,” or “principal angel.” The term occurs in the Bible only in the singular. This seems to imply that there is but only one whom God has designated chief, or head, of the angelic host. Also as already stated, at 1 Thessalonians 4:16 the voice of the resurrected Lord Jesus Christ is described as being that of an archangel, suggesting that he is, in fact, himself the archangel. This text depicts him as descending from heaven with “a commanding call.” It is only logical, therefore, that the voice expressing this commanding call be described by a word that would not diminish or detract from the great authority that Christ Jesus now has as King of kings and Lord of lords. (Mt 28:18; Re 17:14) If the designation “archangel” applied, not to Jesus Christ, but to other angels, then the reference to “an archangel’s voice” would not be appropriate. In that case it would be describing a voice of lesser authority than that of the Son of God.
There are also other correspondencies establishing that Michael is in fact actually the only begotten Son of God. Daniel, after making the first reference to Michael (Daniel 10:13), recorded a prophecy reaching down to “the time of the end” (Daniel 11:40) and then stated: “And during that time Michael will stand up, the great prince who is standing in behalf of the sons of [Daniel’s] people.” (Daniel 12:1) Michael’s ‘standing up’ was to be associated with “a time of distress such as has not been made to occur since there came to be a nation until that time.” (Daniel 12:1) In Daniel’s prophecy, ‘standing up’ frequently refers to the action of a king, either taking up his royal power or acting effectively in his capacity as king. (Daniel 11:2-4, 7, 16b, 20, 21)
This supports the conclusion that Michael is Jesus Christ, since Jesus is in fact Jehovah’s appointed King, commissioned to destroy all the nations at Har–Magedon.—Rev 11:15; 16:14-16.
The book of Revelation (12:7, 10, 12) specifically mentions Michael in connection with the establishment of God’s Kingdom and links this event with trouble for the earth: “And war broke out in heaven: Michael and his angels battled with the dragon, and the dragon and its angels battled. And I heard a loud voice in heaven say: ‘Now have come to pass the salvation and the power and the kingdom of our God and the authority of his Christ, because the accuser of our brothers has been hurled down . . . On this account be glad, you heavens and you who reside in them! Woe for the earth and for the sea.’” Jesus Christ is later depicted as leading the heavenly armies in war against the nations of the earth. (Rev 19:11-16) This would mean a period of distress for them, which would logically be included in the “time of distress” that is associated with Michael’s standing up. (Daniel 12:1) Since the only begotten Son of God is to fight the nations, it is only reasonable that he was the one who with his angels earlier battled against the superhuman dragon, Satan the Devil, and his angels.
In his prehuman existence Jesus was called “the Word.” (Joh 1:1) However, he also had the personal name Michael. By retaining the name Jesus after his resurrection (Ac 9:5), “the Word” shows that he is identical with the Son of God on earth. His resuming his heavenly name Michael and his title (or name) “The Word of God” (Re 19:13) ties him in with his prehuman existence.
The very name Michael, asking as it does, “Who Is Like God?” points to the fact that Almighty Jehovah God is without like, or equal, and that Michael his archangel (Chief Angel) is his great Champion or Vindicator.
All of the aforementioned is where we who are Jehovah's Witnesses derive the doctrine that Jesus was and now is Michael the Archangel.
Those who cling to the unscriptural belief that Jesus Christ is in fact Jehovah God THE Almighty Himself, cannot and will not accept this doctrine because by doing so they would of necessity have to come to both believe and accept that rather than being THE Almighty God, Jesus Christ IS NOT THE Almighty God, but rather, the IMAGE of THE Almighty God, God's ONLY BEGOTTEN SON who is the only thing in all creation that was produced / begotten DIRECTLY by THE Almighty God Himself, while all other things in existence was produced by THE Almighty God THROUGH His only begotten Son (Proverbs 8:30-32; 1 Corinthians 1:24, 30; John 3:16, 20:31; Colossians 1:15; Revelation 3:14).
~ NOTE: I am a dedicated and baptized member of the worldwide unified Christian Congregation of Jehovah's Witnesses.