Below is an excerpt from his talk. Footnotes have been omitted -- see the original.
One of the arguments often used in any defense of a closed canon is the New Testament passage recorded in Revelation 22:18: “For I testify unto every man that heareth the words of … this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book.” However, there is now overwhelming consensus among virtually all biblical scholars that this verse applies only to the book of Revelation, not the whole Bible. Those scholars of our day acknowledge a number of New Testament “books” that were almost certainly written after John’s revelation on the Isle of Patmos was received. Included in this category are at least the books of Jude, the three Epistles of John, and probably the entire Gospel of John itself. Perhaps there are even more than these.
But there is a simpler answer as to why that passage in the final book of the current New Testament cannot apply to the whole Bible. That is because the whole Bible as we know it—one collection of texts bound in a single volume--did not exist when that verse was written. For centuries after John produced his writing, the individual books of the New Testament were in circulation singly or perhaps in combinations with a few other texts but almost never as a complete collection. Of the entire corpus of 5,366 known Greek New Testament manuscripts, only 35 contain the whole New Testament as we now know it, and 34 of those were compiled after a.d. 1000.
The fact of the matter is that virtually every prophet of the Old and New Testament has added scripture to that received by his predecessors. If the Old Testament words of Moses were sufficient, as some could have mistakenly thought them to be, then why, for example, the subsequent prophecies of Isaiah or of Jeremiah, who follows him? To say nothing of Ezekiel and Daniel, of Joel, Amos, and all the rest. If one revelation to one prophet in one moment of time is sufficient for all time, what justifies these many others? What justifies them was made clear by Jehovah Himself when He said to Moses, “My works are without end, and ... my words ... never cease.”
One Protestant scholar has inquired tellingly into the erroneous doctrine of a closed canon. He writes: “On what biblical or historical grounds has the inspiration of God been limited to the written documents that the church now calls its Bible? … If the Spirit inspired only the written documents of the first century, does that mean that the same Spirit does not speak today in the church about matters that are of significant concern?” We humbly ask those same questions.
Continuing revelation does not demean or discredit existing revelation. The Old Testament does not lose its value in our eyes when we are introduced to the New Testament, and the New Testament is only enhanced when we read the Book of Mormon: Another Testament of Jesus Christ. In considering the additional scripture accepted by Latter-day Saints, we might ask: Were those early Christians who for decades had access only to the primitive Gospel of Mark (generally considered the first of the New Testament Gospels to be written)—were they offended to receive the more detailed accounts set forth later by Matthew and Luke, to say nothing of the unprecedented passages and revelatory emphasis offered later yet by John? Surely they must have rejoiced that ever more convincing evidence of the divinity of Christ kept coming. And so do we rejoice.