Was John (who wrote Revelation) a Trinitarian? Absolutely.
Of course, he didn’t formulate the Trinity like later Nicene theologians would, but consider how he opens his letter to the seven churches:
Grace to you and peace from him who is and who was and who is to come, and from the seven spirits who are before his throne, and from Jesus Christ the faithful witness, the firstborn of the dead, and the ruler of kings on earth. (Rev 1:4–5)
John opens his letter by saying that “Grace … and peace” come from three somethings.
“from him who is and who was and who is to come”*
“from the seven spirits”
“and from Jesus Christ”
John’s greeting is intensely triune. Grace and peace come from God, from the seven spirits, and from Jesus Christ. They come from the Three. Divine goodness, namely, grace and peace come through a threefold “fromness.”
John seems to emphasize that by his repetition of the word “from.”
Sometimes it’s easy to think Trinitarian thinking is a later import into Christianity. It’s not. It’s part of the fabric of the New Testament. Again, I don’t mean the later formulation of one substance and three persons. I mean the idea that God somehow worked triunely and that Jesus was included in the being of God (see what I wrote yesterday).