John wrote the book to 1st century Christians, specifically members of seven churches
"Write in a book what you see and send it to the seven churches, to Ephesus, to Smyrna, to Pergamum, to Thyatira, to Sardis, to Philadelphia, and to Laodicea."
The historical purpose of John's vision/dream, yes a terribly scary vision at that, is to reassure the members of these seven early Christian communities that Christ is with the saints and victorious in the end.
"See, the home of God is among mortals.
He will dwell with them;
they will be his peoples,
and God himself will be with them;
he will wipe every tear from their eyes.
Death will be no more;
mourning and crying and pain will be no more,
for the first things have passed away."
In the final section, Epilogue, John and the Angel refer again to the central message of the book: those already persecuted by evildoers can come to Jesus:
"It is I, Jesus, who sent my angel to you with this testimony for the churches. I am the root and the descendant of David, the bright morning star.’
The Spirit and the bride say, ‘Come.’
And let everyone who hears say, ‘Come.’
And let everyone who is thirsty come.
Let anyone who wishes take the water of life as a gift."
I like the emphasis on free choice - "anyone who wishes" may come to Jesus. "Let anyone who wishes take the water of life as a gift."
Early Christians were persecuted, tortured by Greeks/Romans. The vision of John prophesies an even more hellish turmoil than what they are currently experiencing, to which Jesus is still victorious. Jesus is victorious in the vision - his Revelations. Christians will be victorious against their persecutors, in this life - or the next.
The effect of John's book on these early 7 churches would be to give them courage to stay holy, stay the course, Jesus is with them, and his gifts are for them if/when they choose to come to him.
It is not God that is persecuting first century Christians, but John passing on an - albeit scary - invitation to come to Him. Many Christians suggest that God could wipe us all out with great suffering if He wanted to. Point is this - He doesn't want to wipe us out. It didn't happen in the first century, it never will. It happens in John's vision to show that Jesus shall overcome. No matter how our end comes, we will always find the gift that brings us to Jesus. Yes the dreams/visions sounds as more of a warning to early Christians than an invitation, but that is John's Revelation - John's style. The Gospels and Letters from Paul (from St. Jerome's writings and translations by the way) form a much more consistently loving and forgiving - inviting - God for sure.