This passage says many things at the same time.  The first thing to notice is to whom it is addressed- “O ye that dwell on earth”, which obviously  includes everyone. The second element to notice- and it is a distinctive feature of Baha’u’llah’s words- is the majestic and authoritative tone with which He speaks. Baha’u’llah claimed to be the fulfillment of the revelations of the past- the Voice of God- and all His writings after His formal declaration have that tone.  “God’s holy Book”‘ refers to, not just His own words, and not just the Bible and the Quran- but the sum of all of that.  The Revelations of God are part of One Religion, so the “God’s holy Book” is a transcendent reality that includes all of the scriptures of humankind. Baha’u’llah is telling us that whatever is written in the Holy Books of the past has now been explicitly revised by God- to take out whatever might have been the source of disunity and division, as the fundamental purpose of God’s revelation in this day is unity.  Only God can edit his own book, and Baha’u’llah is explicitly doing just that to ensure that religion now becomes a source of unity. Pretty amazing stuff!