Is Rev. 19:9–10 sufficient to tell us not to worship an angel ?
By the time Revelation was written, Christianity already spread to many pagan Gentile nations, practicing some form of polytheism or another, in which both the existence and the worship of many gods was a given; one relevant episode is depicted in the book of Acts:
Acts 14:11-15 And when the people saw what Paul had done, they lifted up their voices, saying in the speech of Lycaonia:
The gods are come down to us in the likeness of men.
And they called Barnabas, Jupiter; and Paul, Mercury, because he was the chief speaker. Then the priest of Jupiter, which was before their city, brought oxen and garlands unto the gates, and would have done sacrifice with the people.
Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out, and saying:
Sirs, why do ye these things? We also are men of like passions with you, and preach unto you that ye should turn from these vanities unto the living God, which made heaven, and earth, and the sea, and all things that are therein.
It is thus not for John's sake, but for that of his audience, that the angel draws attention to his innocent (and, in my opinion, legitimate) gesture of reverence.
The curtain of the Jerusalem temple was luxuriantly adorned with images of golden cherubs (Exodus 26:1), as was the ark of the covenant (Exodus 25:18-21):
thus, when pious Jews eventually entered the Temple, they would inevitably face that majestic curtain, and its sumptuous angelic imagery; as they would fall down on their faces, to worship God, they would logically do so facing said curtain, and its decorative artwork;
similarly with Moses, bowed down on his face before the ark, as God would reveal Himself to him there, by appearing between its two cherubic statuettes (Exodus 25:22).
The apostle of Christ did nothing else in the celestial sphere, than what countless Hebrew priests and prophets and the pious faithful have been doing for millennia in its earthly counterpart, obeying the Torah's express directive.
But a large part of John's diverse audience wasn't necessarily intimately familiar with its monotheistic tenets, and its various cultic or ritual subtleties; to avoid any possible misunderstandings on behalf of his non-Jewish readership, the angel states things plainly, not just once, but twice; in other words, whenever due reverence is shown unto him by the otherwise unsuspecting apostle, repeatedly hammering the fine point down, for all to see and hear.
I am a fellow servant with you and your brothers who hold to the testimony of Jesus.
Christ came down from heaven to lift up his (and, implicitly, our) human nature at the right hand side of God the Father, through the ascension to heaven, following his resurrection from the dead (Matthew 26:64; Mark 14:62, 16:19; Luke 22:69; Acts 2:33, 5:31, 7:55-56; 1 Peter 3:22).
Thus, as when previously, before the incarnation, humans were indirectly worshipping angels, since their holy images were placed between them and the place where God dwelt, in the earthly temple or tabernacle, as shown above, so now, after His celestial ascension, angels, in turn, are indirectly worshipping humans, when bowing down to Him and the incarnate word, standing on His right hand side, beside the heavenly throne.
Thus, the dialogue between the two worlds, taking place in the quoted passage, is somewhat curious:
on one hand, the human being is somewhat perplexed as to the angel's (to him, incomprehensible) reaction;
on the other hand, the angel, in turn, stands somewhat perplexed by the human's (to him, deeply puzzling) action.
This baffling or unsettling upheaval in well-worn norms and pre-established conventions is the direct consequence or aftereffect of Jesus' incarnation-and-ascension; indeed, it reflects a relatively similar exchange, between John and his cousin, as recorded in the synoptic gospels:1
Matthew 3:13-15 Then cometh Jesus from Galilee to Jordan unto John, to be baptized of him. But John forbad him, saying:
I have need to be baptized of thee, and comes thou to me ?
And Jesus answering said unto him:
Suffer it to be so now: for thus it becomes us to fulfil all righteousness.
Then he suffered him.
As then it was once utterly intolerable for the greatest man born of a woman (Matthew 11:11, Luke 7:28) to witness the one who will eventually baptize mankind with heavenly fire, humbly coming to be baptized by him in earthly water, so here, it is likewise equally insufferable for the angel to see man, whom he now worships daily in heaven (Hebrews 1:6), to fall down before him, as if he, the angel, would (still) stand in need of reverence and admiration of him (man), rather than the other way around, thereby generating, all of a sudden, all manner of confusion to both persons involved, because the former was simply following a well-established, divinely instituted, ancestral custom; whereas the other was already painfully aware that now the rules have dramatically changed, being turned on their head, upside-down, by the Revelation of the Messiah.
1 I can't help but wonder, and ask in return, whether posting this particular question on the day commemorating Christ's baptism by John (January 6) was intentional, or just a freak coincidence ?