The first part of this verse affirms the general truthfulness of the biblical text, except for the contrast between its “plain and pure” origin and, by implication, a less perfect history. The comment that John’s book had originally been “easy to the understanding of all men” amazes modern readers who struggle to comprehend the book of Revelation. It is currently anything but “easy to the understanding.”
Has the text has been altered? Apparently not. The first manuscripts of John date to the second century, and there is no evidence that the text has undergone anything other than the normal transmission process. The meaning has become opaque, rather, because our different culture limits our ability to understand the text.