He which testifies these things said, Surely I come quickly. Amen. Even so, come, Lord Jesus. [Revelation 22:20]
The book of Revelation foretells disasters, plagues, wars, evil rulers, and rise of the antichrist before Christ comes again to establish His millennium kingdom. Then after that, there will be a final war with the evil one, and a large-scale judgement, before the new heaven and new earth appear.
In brief, terrible things will happen before Christ comes again. Despite that, John responds “Even so, come, Lord Jesus.” Matthew Henry explained that it was because John knew that Christ’s coming would put an end to sin, sorrow, and temptation, and establish perfect purity, peace, and joy. That was something the church was hoping for. I think that it was also because John had faith in Him that He will protect His flock from harm.