Perhaps it's significant that the last time John had seen his Lord, Jesus Christ, it was as he ascended bodily up into the clouds, to then disappear from sight. Now, decades later, John sees Christ in his ascended glory. He is initially described as the Son of Man, and John recognises him as such. He testifies to this Son of Man as being "the first begotten of the dead", whose love, and cleansing from his sins John has personally experienced (1:5). This assures John at the outset as to whose revelation he is receiving - God's revelation given to Jesus Christ, the Son of Man who he spent years with, when on earth.
But the revelation shows John the unimaginable glory of the risen Christ. First, he hears an immense sound as of a trumpet, but it is a great voice (vs. 10). He has to turn and look behind him, from where the sound emanated. He sees in vision seven golden candlesticks, in the midst of which is "one like unto the Son of man" (vs. 13). But this Son of man is unlike anything he saw Jesus as, when on earth. Not even the transfiguration appears to equal this risen Christ! Language used to describe God himself is applied to Christ (vss. 14-16). The sight of this glorified Christ, as Son of Man, causes John to fall at his feet as dead (vs. 17).
This means that John has been swiftly prepared to enlarge his understanding of Christ, to see him in terms of the First and the Last, who has the keys of hell and of death (vs. 18). Then comes instructions regarding the seven churches. Christ is not mentioned as being the Son of Man at any point in that entire section where the seven churches are being addressed. And note, too, that this is all what "the Spirit says unto the churches" (relayed via an angel).
Only regarding the church at Thyatira is Christ designated. Verse 18 says, "These things saith the Son of God..." and then REPEATS the description of him having eyes like flame of fire, and feet like burning brass, which was first stated in 1:14-15. Just as John needed that vision of the awesome glory of the risen Christ at the outset, so Thyatira in particular needs to be reminded. They would have received the first verses of that vision of glory as well as the letter to them. Perhaps Thyatira was prone to forgetting the glory of their Lord, and needed to be reminded that the Son of God saw right through them, so that he could expose "that woman Jezebel" as well as commend those who did not tolerate her prophecies, fornications, and idolatries. Christ referred to that as "the depths of Satan (vs. 24). The rest of the vision exposed the full monstrous horror of the depth Satan goes to to corrupt Christ's church, and the faithful ones in it. But this church had allowed satanic depths to come right inside! So we see Christ speaking as Son of God, who will judge such satanic evil inside his own Church. "For judgment begins with the household of God" (1 Peter 4:17).
Yet Thyatira is assured that the faithful ones will not be so treated as the 'Jezebel' element trying to corrupt the rest. This church needs to look around and to see above them the Son of God, who is also the Son of Man. The other churches do, too, but it seems as if Thyatira could be more corrupted than the rest (indeed, some of them are not corrupted at all, and are commended.) But the point of significance is that the Son of Man in chapter 1 is described the same as is the Son of God in chapter 2. This proves that Christ is both Son of Man and Son of God.
