You should not be certain since all you have is belief. You in effect just admitted to thinking irrationally. Not a good start to a debate. And those that accepted Revelation had no way of testing to see if John actually wrote it either so your conclusion is not merited. You appear to be a huge fan of circular reasoning. Now the fact is that modern scholars who study the works much more thoroughly than you or I do have concluded by the different writing styles that Revelation was not written by the same author as John, and none of them think that John was written by the apostle John.
And there are some very good reasons to doubt that John wrote it. First it was written at the earliest about 90 CE and possibly after 100 CE. John the apostle would most likely have been dead at that time. Second it was written by a well educated man and none of the apostles were thought to qualify for that.
Do you have anything besides "it mush have been" claims? Real scholars, not apologists who tend to be liars for the Bible, are not reliable resources. Those that know the history of the Bible know that the names were given much later and at that time famous names were often attached to works to give them a sense of veracity.
EDIT: Though not the definitive source Wikipedia is often a very good place to start, they not only have good articles, they have links to their sources:
Gospel of John - Wikipedia
But here are a few more:
The Story Of The Storytellers - An Introduction To The Gospels | From Jesus To Christ | FRONTLINE | PBS
This article points out the signs that it was written by several people:
Gospel According to John | Description, Authorship, & Facts