I recently heard that a prominent Adventist author teaches the idea that the Father created Jesus a long time ago, then promoted Him into the Godhead. Furthermore, Lucifer became jealous because he wanted to be promoted as well.

Anybody have any clue who is teaching this?

No --
But the idea that God "brought forth" a Son then promoted Him into the Godhead and Lucifer became jealous because he wanted to be promoted as well is very much part of the those pushing for a anti-trinitarians.

What I see as the "root" of the problem is people ascribing to Jesus Original status things which He took upon Himself as He desended down to meet His created creatures.

Jesus was the One Who was in the Form of God, Who was equal with God, One with God, Who was God. He was the ONE WHO made himself of no reputation, and took upon himself the form of a servant, and was made in the likeness of men (See John 1:1, Phil. 2:6)

After He had suffered the fullest disgrace in His death upon the cross, because of OUR TRANSGRESSIONS, Jesus Christ, Who will forever carry His identification with humanity, (the SON OF MAN as well as the SON OF GOD) was escorted to heaven and exalted as the King of the glory. There in heaven He took His place with the Father on His throne-- the only One worthy to implement the grand covenant between God and man."

From the highest point, He descended to the lowest depths. He accepted willingly a dependent relationship with God the Father, the same relationship, which humans may have to God.

And when He returned to heaven, He was hightly exalted!

But His Exaltation wasn't a "promotion" in the sense anti-trinitarians seem to teach.

Oh, someone says -- But He was exalted prior to creation, doesn't that mean He was "promoted".

But look at it this way.
Christ is called the "archangel" -- Why. HE IS NOT AN ANGEL. But He goes forth and mingles with His Created beings. He is the COMMANDER of the angels. The Lord of Hosts! It is in this sense that Lucifer was "second in command" as a leader of the angels. The idea that Lucifer and Christ were on the same level at any point in time is plain blasphemy.

But when Lucifer began to agitate this idea, God the Father called an assembly and showed them Christ's true exalted position.

"The King of the universe summoned the heavenly hosts before Him, that in their presence He might set forth the true position of His Son" [PA 36]
"There had been no change in the position or authority of Christ. Lucifer's envy and misrepresentation and his claims to equality with Christ had made necessary a statement of the true position of the Son of God; but this had been the same from the beginning. Many of the angels were, however, blinded by Lucifer's deceptions. {PP 38.1}

So that "exaltation" was not a promotion or "change in position or authority of Christ. It was to show the angels that their Leader was equal with God! Was indeed God.

Lucifer became jealous of Jesus because he wasn't invited to help design the human race. There is zero mention of him wanting to be promoted to divinity.

Isaiah 14:12-14 KJV How art thou fallen from heaven, O Lucifer, son of the morning! how art thou cut down to the ground, which didst weaken the nations! 13 For thou hast said in thine heart, I will ascend into heaven, I will exalt my throne above the stars of God: I will sit also upon the mount of the congregation, in the sides of the north: 14 I will ascend above the heights of the clouds; I will be like the most High.

SDABC: Like the most High. Lucifer aspired to be like God in position, power, and glory, but not in character. He desired for himself the homage the angelic host gave to God. Only a created being, he sought honor due alone to the Creator. Instead of seeking to make God supreme in the affections of the angelic host, he sought for himself first place in those affections.

EGW: Satan's Rebellion of Long Standing.--The records of some are similar to that of the exalted angel who was given a position next to Jesus Christ in the heavenly courts. Lucifer was enshrouded with glory as the covering cherub. Yet this angel whom God had created, and entrusted with power, became desirous of being as God. He gained the sympathy of some of his associates by suggesting thoughts of criticism regarding the government of God. This evil seed was scattered in a most seducing manner; and after it had sprung up and taken root in the minds of many, he gathered the ideas that he himself had first implanted in the minds of others, and brought them before the highest order of angels as the thoughts of other minds against the government of God. Thus, by ingenious methods of his own devising, Lucifer introduced rebellion in heaven. {4BC 1143.1}
God desired that a change take place, and that the work of Satan be brought out in its genuine aspect. But the exalted angel standing next to Christ was opposed to the Son of God. The underworking was so subtle that it could not be made to appear before the heavenly host as the thing that it really was; and so there was war in heaven, and Satan was expelled with all who would not stand on the side of loyalty to God's government. The Lord God stood forth as Supreme Ruler. {4BC 1143.2}
This condition of things had existed a long period of time before Satan was unmasked and the evil ones expelled (Letter 162, 1906). {4BC 1143.3}

I think nowadays it would not be possible for a prominent Adventist author to teach this and still retain his functions.

So Christ was just a angel or created being like Adam, and worthy to be worship. I think I see a problem with that.....

True, Ellen White & James White would be disfellowshipped.

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But she would not be surprised because the Harmon family were treated the same by the Methodist Church.

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Basically you are saying that Christ and Lucifer were on the same level. Both created beings. Christ was exalted to divinity but Lucifer wasn't thus he had good reason to rebel.

If EGW taught that (WHICH SHE DIDN"T)
then there would be no choice but to discard her writings as inspired.

To tell you the truth I am absolutely shocked at some of the things people say here about Our Lord and Savior.

I'm sure Satan is laughing with glee. It is his purpose to contest the true nature of Christ as fully God in the fullest sense.