Deuteronomy 18:20-22 reads:
20 But the prophet, which shall presume to speak a word in my name, which I have not commanded him to speak, or that shall speak in the name of other gods, even that prophet shall die.
21 And if thou say in thine heart, How shall we know the word which the Lord hath not spoken?
22 When a prophet speaketh in the name of the Lord, if the thing follow not, nor come to pass, that is the thing which the Lord hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
This passage has enjoyed great popularity among Christians attempting to demonstrate that Joseph Smith was a false prophet. There are a number of relevant issues that could be discussed from a LDS point of view, including scholarly Jewish interpretations of the passage and apparent false prophecies uttered by Old Testament prophets. But for this discussion I’d like to approach the issue from the position that all prophecy (by which, in this discussion, we mean inspired prediction) is contingent on other factors for their fulfillment. Prophecy does not operate in a vacuum, and its accuracy is dependent on factors that are not always initially made obvious.
I’d like to make this point by briefly discussing one example from the Old Testament.
In 1 Sam 2:30, an unnamed prophet appears to Eli, the priest at Shiloh and delivers this message:
30 Wherefore the Lord God of Israel saith, I said indeed that thy house, and the house of thy father, should walk before me for ever: but now the Lord saith, Be it far from me; for them that honour me I will honour, and they that despise me shall be lightly esteemed.
The context of this message is that Eli’s sons have been wicked in carrying out their duties as priests, and Eli has not been diligent in controlling his sons. In generations past their family was promised that their house “should walk before me forever” as the LORD’s designated family of priests, and it doesn’t appear that any provisions were explicitly given at that time indicating that that privilege could be revoked. But here, generations later, the Lord decides to revoke that privilege because of their wickedness.
I submit that this is an example of the contingent nature of prophecy. We can consider the Lord’s statement to Eli’s ancestors that their house (the house of Aaron) would forever “walk before [him]” as prophecy. That prophecy was contingent on Aaron’s family being faithful in their duties as priests. Eli and his sons were not, and so the Lord revoked that privilege.
The prophecy did not come to pass. The Lord’s prophecy did not come to pass. Is that a problem? Well, it is a problem for Eli and his sons, but it isn’t a problem for God. God didn’t fail, Eli and his sons did. Prophecy by nature is contingent and so it isn’t an infallible method for determining prophet-hood. Were it so, God himself would be impeached.
What do we then make of Deuteronomy 18:20-22? It is one method among many given in the Bible for determining if a prophet is a true prophet, and shouldn’t be employed in isolation from the other methods. But that discussion is for another day.
Related reading here: http://en.fairmormon.org/Joseph_Smith/Prophecies/The_prophetic_test_in_Deuteronomy_18