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This is when he says "Let all the angels worship Him": "when he bringeth in the firstbegotten into the world". It is a specific command at a specific time for a specific reason.
Why was it necessary to issue such a command if the 'Him' is the second person of the Godhead whom the angels had always already been worshiping?
This Son of God had done something that the angels do not fully understand. 1 Peter talks about the salvation that was preached through the prophets, who did not fully understand what they were prophesying, as well as the angels desiring to look into these things that have to do with the salvation of man:
Of which salvation the prophets have enquired and searched diligently, who prophesied of the grace that should come unto you: Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow. Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into. - 1 Peter 1:10-12
He who was God in the beginning (John 1:1) and for whom equality with God was no misappropriation (Philippians 2:6) emptied Himself, made Himself of no reputation by taking on the form of a servant, humbled Himself and obeyed unto death, and was raised again for our justification.
He was made a little lower than the angels (Hebrews 2:5-11) in order to raise men to a position of judging the angels (1 Corinthians 6:3). He went from creator of angels to lower than angels when He took on flesh. He became son of man and yet did not cease being Son of God. The angels neither expected nor fully understood such a thing (perhaps they do not fully understand still) since this plan was wrought in the council of the Almighty from before the foundation of the world, from before they were created.
The angels needed instruction that this son of man, this Word tabernacled in flesh, this Jesus, was to be worshiped as always even during His sojourn of humility because they were/are longing to look into these things that they otherwise did not fully comprehend.
When angels appear worshiping God at certain moments in time or in visions etc. it does not mean they are not always worshipping during other periods it just means certain events are highlighted to us in order for us to understand specific truths. Of course angels always worship the Son of God. What Hebrews is getting at is the ‘authority over all creation’ that the ‘man’ Jesus, who was also God in his office of the Messiah, acted with a ‘first-born right’ over all of heaven and earth in the role of that office for us. That this is a much greater glory than the Old Covenant is evidenced in the worshiping of the angels of Christ and not Moses. The worship is in particular respect to the first-born right he obtained by his birth, life, death and resurrection, carrying out that office in glorious victory, on our behalf. The same worship of Jesus appears many times in Revelation. (Rev 5:11, 7:11 etc)
God made David like is first-born in a prefigure of the Messiah’s future office in due time:
Psalm 89:27 (ESV): 27
And I will make him the firstborn, the highest of the kings of the earth.
Some of the early church fathers chose different parts of his office, either birth, resurrection, second coming etc to understand the Psalm but there is no need to debate or spit hairs when it is obvious the whole office as representing a New Covenant as compared to the Old that Hebrews is arguing. Christ is greater than Moses because the Angels worship Him.
A good a thorough explanation can be found in old but famous commentary on Hebrews ‘An Exposition of the Epistle to the Hebrews, Volume 3’ by Owen, John
This, then, is the sum of these words of the apostle: ‘Again, in another place, where the Holy Ghost foretells the bringing forth into the world and amongst men him that is the Lord and Heir of all, to undertake his work, and to enter into his kingdom and glory, the Lord speaks to this purpose, Let all the angels of God worship him.’
An intimation of the pleasure of God unto the angels. Not that divine worship was absolutely due unto the Son of God, which they knew from the first instant of their creation, but that all honour and glory were due unto him on the account of his work and office as mediator and king of his church.