Prophecy is Essential for Faith - Jacob 4:6

Wherefore, we search the prophets, and we have many revelations and the spirit of prophecy; and having all these witnesses we obtain a hope, and our faith becometh unshaken, insomuch that we truly can command in the name of Jesus and the very trees obey us, or the mountains, or the waves of the sea.

 The true saints have always enjoyed the gift of the spirit of prophecy from the very beginning. Amos truly said: Surely the Lord God will do nothing, but he revealeth his secret unto his servants the prophets (Amos 3:7). The spirit of prophecy is vital to the church of Christ. If we look at the Old Testament, nearly every book was written by a servant of God who had the spirit of prophecy. Consider this statement of Christ concerning the legacy and longevity of the prophets and the Jews' treatment of them: ...upon you may come all the righteous blood shed upon the earth, from the blood of righteous Abel unto the blood of Zacharias son of Barachias, whom ye slew between the temple and the altar. (Matthew 23:35).

The spirit of prophecy was not given to the Jews only - don't forget the heathen prophet Balaam who prophesied of the coming of Christ: (Numbers 24:16-24), and Job who prophesied of Him: (Job 19:25-26). There are others as well, the three wise men, for example. After Christ as well, the legacy of prophecy continued with the Apostles, consider the visions Paul, Peter and of Stephen (Acts 7:55-56), and Paul's statements about the organization of the church to include "prophets" (Ephesians 4:11).

Remember that faith is a gift, which "faith cometh by hearing, and hearing by the word of God" (Romans 10:17). And how shall we have the word of God except by the spirit of prophecy?

Consider this gem revealed unto us by Nephi:

Do ye not remember that I said unto you that after ye had received the Holy Ghost ye could speak with the tongue of angels? And now, how could ye speak with the tongue of angels save it were by the Holy Ghost? Angels speak by the power of the Holy Ghost; wherefore, they speak the words of Christ. Wherefore, I said unto you, feast upon the words of Christ; for behold, the words of Christ will tell you all things what ye should do. (2 Nephi 32:2-3)

There are many ways in which God reveals himself (Moroni 7:22-25). But it appears that there are two principle sources of the word: The first being the prophets themselves, whether via the scriptures or directly from their lips; the second being the spirit speaking directly to us. Both of these sources of the word are available to us, but the second one is just as necessary as the first. Because without the spirit to speak truth unto our souls, there is no way to confirm the words of the prophets. But with the spirit, God can reveal the "truth of all things". (Moroni 10:5, John 14:26)