What is the origin of the idea that Michael is Jesus?

There is much negative information on the internet on this subject making it very difficult to earnestly study it and where the belief that the Archangel Michael is Jesus Christ originated.

The Jehovah's Witnesses surely inherrited it from Charles Taze Russell, the founder of the movement that led to the formulation of the Jehovah's Witnesses.

He equates Michael with Christ in one of Daniel's prophecies:

The Time is at Hand Page 145 - Written 1889
The same events are referred to in Daniel’s prophecy: “And at that time shall Michael [Christ] stand up ...

Russell doesn't plainly say "Michael is Jesus" but I think his use of brackets implies it well enough that it was a position he held or was considering.

The Seventh Day Adventists surely inherrited this bit of theology from Ellen G. White, their prophetess, whom they hold in high regard.

She plainly states in several of her writings that Michael is Jesus.

Patriarchs and Prophets Page 761 - Written 1890
Again: Christ is called the Word of God. John 1:1-3. He is so called because God gave His revelations to man in all ages through Christ. It was His Spirit that inspired the prophets. 1 Peter 1:10, 11. He was revealed to them as the Angel of Jehovah, the Captain of the Lord's host, Michael the Archangel.

The Desire of Ages Page 99 - Written 1898
The words of the angel, "I am Gabriel, that stand in the presence of God," show that he holds a position of high honor in the heavenly courts. When he came with a message to Daniel, he said, "There is none that holdeth with me in these things, but Michael [Christ] your Prince."

Prophets and Kings Page 572 - Written 1917
For three weeks Gabriel wrestled with the powers of darkness, seeking to counteract the influences at work on the mind of Cyrus; and before the contest closed, Christ Himself came to Gabriel's aid. "The prince of the kingdom of Persia withstood me one and twenty days," Gabriel declares; "but, lo, Michael, one of the chief princes, came to help me; and I remained there with the kings of Persia."

Neither White nor Russell were the first to say this. There is much disinformation claiming that the concept is an old Arian doctrine, however, there is no evidence for that in the history books, and I can find no evidence that it predates the 18th century, though you found John Calvin discussing it in the 16th century.

The oldest source I can find on this subject is "The Glory of Christ as God-Man" by Isaac Watts. The linked source below was printed in 1795, but Watts died in 1748, so he clearly penned it earlier than 1748.

When examining the story of Daniel in the Lion's Den, Watts suggests it as a possibility that the Archangel Michael is Christ on page 223

The Glory of Christ as God-Man Page 223 - Written before 1748 from an edition printed 1795
And may not Christ himself be this Michael the Arch-angel, the Prince of Israel? It has been observed by some writers, that the scripture never speaks of arch-angels in the plural number; perhaps there is but one arch-angel and that is Christ.

Interestingly, in a footnote on page 224, Watts notes that some think that the angel that appeared to Daniel was not Gabriel, but was Christ. Also interesting, Watts does not seem to be giving his own theology, but parroting what he has read or heard elsewhere. The fact that John Calvin was discussing it 200 years earlier is evidence of that.

Moving on with another, pre-White, pre-Russell source ...

A man named William Kinkade wrote "The Bible doctrine of God, Jesus Christ, the Holy Spirit, atonement, faith, and election" in 1829, predating White and Russell by over 50 years. I cannot find any information on this particular man except that he wrote this book. In it, he dedicates an entire chapter to Michael and Jesus and the theology that they are the same person.

The Bible doctrine of God, Jesus Christ, the Holy Spirit, atonement, faith, and election Page 153 - Written 1829
To me this evidence proves beyond resonable dispute, that Michael is one of the names of Christ; because if the Church is the seat of this war, and if Christ is the Captain of our salvation, and the leader of his people, he must be the person who is here mentioned under the name of Michael.

Kinkade is not the only person of this time to hold this view, he claims.

ibid Page 152
I am not alone in this opinion; most of the principal writers of the Trinitarian school have advocated the same doctrine.

Kinkade goes on to mention more than a few sources that support his theology. Here are some that go beyond calling Jesus an Angel and actually call Jesus and Michael the same person, according to Kinkade [the numbers are mine, but the text is Kinkade's]:

ibid Page 152 and 153

1. Brown’s dictionary of the Bible on the words Michael, and Angel says, that both these words do sometimes refer to Christ; and also affirms that Christ is the archangel.
2. Woods Spiritual Dictionary teaches nearly, if not exactly, the same on this subject that Brown’s does.
3. Butterworth, Cruden, and Taylor in their concordances, assert that Michael and Angel are both names of Christ.
4. Guyse in his Paraphrase on the New Testament, on Rev. 12:7, acknowledges that many good expositors think that Christ is signified by Michael; and also gives it as his opinion.
5. Doctor Watts in his Glories of Christ, pp. 200-202, 218, 223, and 224, teaches the same doctrine.
6. Thomas Scott, in his notes on the Bible ... asserts that Michael the Archangel is Jesus Christ.

The numbers below are for a brief description on the above sources listed by Kinkaid.

1. Wikipedia's article on John Brown lists his dictionary as first in print in 1769 and having numerous editions through 1868. This is the earliest edition I could find dated 1866. There is really no way of knowing which edition Kinkade was referring to.
2. I cannot find a "Spiritual Dictionary" written by someone named "Wood".
3. John Butterworth's Concordance is from 1812. Curden's Condordance, written by Alexander Cruden is around 1849. The edition that Kinkade used could be any one of many. John's Taylor's concordance is an Hebrew concordance. I cannot find an online source.
4. Guyse's paraphrase is in several volumes and the one linked does not seem to contain the related parts (Revelation). Wikipedia lists the dates for the volumes as 1739-52.
5. Isaac Watts was quoted above as the earliest source found. He had a desire to bring the Church to an Arian view, which may be the reason that there is confusion that the "Michael is Jesus" perspective was originally an Arian perspective.
6. Thomas Scott began writing his commentary (or notes, as Kinkade called them) in 1788. Searching the source linked does not reveal anything mentioning Michael. It may be the wrong source, or perhaps his notes on that subject have not survived.

Now whether White or Russell learned this theology from Kinkade or another source is unknown. We do know that both were very well read so they may have learned if from any of them. They also may have independently conceived it, though I doubt it.

Observations on the development of this theology

John Calvin in the 16th century mentions that "some think" Michael is Jesus then discusses it a bit further, but he clearly denies to hold that theology himself and further notes that it "is not generally admitted." Watts is also careful to flatly claim it as his own theology, but is clearly not hesitant to discuss it at length. It is not until the 19th century that various theologians began claiming it as their personal belief, while discussing it, and sometimes, in the case of Kinkade and White, even preaching it to convince their readers.

So what are we to make of this? The only option that makes sense is that no prominent theologian was willing to discuss this idea until Calvin, though he wrote at a time when "heretics" were still being burned. Before Calvin, I suspect it was something that insignificant or amateur theologians discussed quietly among themselves to avoid persecution and accusations. But, slowly, as the Catholic church lost its grip on the stately matters of its Church-State, theologians holding this view came out of the shadows, no longer fearing persecution for the implications that this theology makes on the nature and divinity of Christ.

But this still leaves the original question: Where did this theology originate? As it sits now, we do not know. But by my reasoning above it surely originated before Calvin, must have been somewhat known among theological circles, but quietly discussed, and was not openly professed until the 19th century. Because we cannot find any other theologians before Calvin mentioning it, or even the Catholic Church condemning it, I find it difficult to believe that it would remain a quiet or secret matter for more that 200 years before Calvin. I reserve the right, however, for the history books to disprove me.

Conclusion

It would seem that the earliest surviving work to mention this subject is from the early 16th century. It does seem quite possible, however, that less popular theologians were teaching that the Archangel Michael is Jesus Christ before that time. Though, possible, however, the only evidence is Calvin's word that "some think". Even today, the belief is not common among Christians and is considered exclusive to non-trinitarian sects, such as the Jehovah's Witnesses, but this is not true, Seventh Day Adventists being classic trinitarians. These two groups together, make a meager 25 million persons among the billion plus Christian population.

Notes on the differences that JW's and SDA's look at this bit of theology.

The Witnesses would say that Michael is Jesus to support their belief that Jesus is not the Almighty God, but rather a god, and He has manifested Himself under the name Michael at times. To the Witness, they cannot reconcile Michael and Jesus being the same person and Jesus being the eternal God. Adventists can and do.

Adventists (I use this term to mean the modern SDA's) are classic trinitarians. They do, in fact, believe in the full divinity of Christ (though there is some evidence that there progenitors did not). And at the same time they believe that Michael the Archangel is actually Jesus Christ under a different name. They see no reason that the view that Michael is Jesus should affect the divinity of Christ.I have written more on this and much more on Adventists in general here: What are the main differences between 7th Day Adventists and Catholic/Protestant churches?

I feel compelled to give some credit to this blog post. Without it I would not have found Kinkade's work, which means I would not have found Watts' work either.