"For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:" - Revelation 22:18
This scripture was written by John the Revelator, on the Isle of Patmos, sometime during the latter end of the 1st century. Now whether or not other sacred texts were written after is not relevant to my argument today.
It is a fact that the Books in the Bible were compiled together during the 4th century shortly after the Nicene Council. If this is true, then prior to this, the book of Revelation itself stood alone and during the compilation of the Bible those other Books were added to the Book of Revelation, then. This would mean, by those who wish to demean and discredit the Book of Mormon with this gross misinterpretation of this individual scripture, would condemn their own sacred texts and by their own standards blatantly declare that the Bible itself is a spawn of sin and was compiled contrary to the commandments of God. Therefore, those who wish to pursue this argument further must decide either John the Revelator was a liar and was not inspired of God, discrediting the entire Bible itself, or that scripture speaks exclusively to the Book of Revelation.
The Book of Mormon is the Word of God just as much as the Bible.