Revelation 19:10

And I fell at his feet to worship him
Being transported with the news he brought him of the marriage, or conversion of his countrymen the Jews, and struck with reverence and awe of the glory and majesty in which the angel appeared to him; and forgetting himself, that worship was only due to God, he behaved in this manner; which is not to be excused nor justified, as appears from the angel's words:

and he said unto me, see thou do it not;
the words are in the original very short and concise, and are spoken in an abrupt manner, and in great haste; as fearing he would be guilty of idolatry, before he could speak all his mind, and use the arguments that were necessary to dissuade from it:

I am thy fellow servant, and of thy brethren that have the testimony
of Jesus;
if this was one of the ministering spirits, he was a servant of the same Lord as John; and if he was a minister of the Gospel, he was still more literally a fellow servant of his, and of the apostles, and preachers of the Gospel; which is meant by the testimony of Jesus, that bearing testimony to the person, office, grace, obedience, sufferings, and death of Christ, and the glory following; and therefore being but a servant, and a servant in common with John and his brethren, was by no means to be worshipped; not the servant, but master; not the creature, but the Creator:

worship God
and him only, even God the Father, Son, and Spirit; not the Father to the exclusion of the Son, the firstborn, whom all the angels are called upon to worship; nor of the Spirit, who is equally joined with the Father and Son in baptism, a part of religious worship, and in other parts of it also; but this excludes all creatures, angels, and men, things animate or inanimate, and images of them; the worshipping of which will now be no more, or at least will be quickly at an end.

For the testimony of Jesus is the spirit of prophecy
that is, the testimony of Jesus, or the Gospel which John and his brethren had, is the very spirit, life, and soul of the prophecy of this book; for as all the prophets bore witness to Christ, so does the Spirit of God in this; or the testimony which they had, and bore to Christ, was equal to the spirit of prophecy with which this angel was endowed; so that he and they were upon an equal foot; and he was no more a proper object of divine and religious adoration than they were.