.Question;
IV. Truth without any mixture of error
Prov 30:5 Every word of God is pure: he is a shield unto them that put their trust in him. Prov 30:6 Add thou not unto his words, lest he reprove thee, and thou be found a liar." Were not the entire works of the New Testament added words which were written long after this was written? And if they are also inspired by the Holy Spirit, how can we be certain that later writers such as Joseph Smith are not equally inspired?

Answer: If this passage is interpreted in such a way as to invalidate all writings after that of Solomon, then a major part of the Old Testament must be discarded since all the prophets of the later kings, the captivity and the rebuilding of the nation came later. This would certainly rule out all modern day ‘prophets’ as well.
On the other hand, if it is rightfully admitted that it leaves room for the New Testament and that the New Testament is inspired, then all modern prophets, such as Joseph Smith, are still excluded for the very close of the New Testament proscribes any further inspired writings.

Rev 22:18 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:
Rev 22:19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.

Prov 30:6 Add thou not unto his words, lest he reprove thee, and thou be found a liar. This verse neither says nor implies that God, Himself, would not add to His words. John, on the other hand was very specific. He designated ‘THIS BOOK’. When John penned the final ‘Amen’ to the Book of the Revelation, the inspired Word was complete. Thus, anything that comes later is spurious.