Why prophets?
Modern truth is given to us by modern prophets.
Mormonism is unique among Christian churches in that it not only accepts the existence of prophets today, but is led by a prophet. Although Mormons believe in the Bible and put great weight on its teachings, Old and New Testament, although Mormons believe, also, strongly in another ancient work of scripture, the Book of Mormon - although they believe in these as the Word of God, they also believe revelation did not end with them.
They believe, instead, the revelation thrives today and that God still has much to tell us and will continue to speak to us so long as the world persists. The Mormon Church is, again, guided by a prophet, who is guided by the Lord. The Mormon Church has always been guided by prophets, since its founding in 1830 by Joseph Smith. Each of these prophets has revealed more truth unto the Church and the world.
Why are modern prophets necessary? Ancient scripture tells us a great deal about the Lord's word centuries ago, and is still important and relevant today. However, the Bible cannot always give us specific guidance about the troubles of the modern world, which are not always the same as those of the ancient world.
What is revelation? Prophets receive the word of God through revelation - communication from God. But not only prophets commune with God - God can give each of us individual guidance through the Holy Spirit.
Who was Joseph Smith? Joseph Smith was the first prophet of the Mormon Church and its founder. Through him, the Lord restored many lost truths to the world.
What did Joseph Smith teach that was so unique? The Lord revealed to Joseph Smith certain essential principles and tenets of Christianity, such as temple worship and the importance and eternal nature of families.
Who leads the Mormon Church today? Gordon B. Hinckley is the fifteenth prophet of the Church and as guided by the Lord as his predecessors.