Joseph Smith restored teachings from the early Christian church that were later declared as heresies, such as premortal life and deification.
Speaking of the pre-existence, William de Arteaga wrote:
This question was hotly debated by Christians of late antiquity, and the faction of the Church which was bitterly opposed to preexistence gained the upper hand. By the sixth century belief in preexistence was declared heresy. All of this is quite astonishing in view of the clear and repeated biblical evidence for preexistence.– William de Arteaga, Past Life Visions: A Christian Exploration (New York: Seabury Press, 1983), 127
Speaking of deification, Clement of Alexandria wrote:
yea, I say, the Word of God became a man so that you might learn from a man how to become a god– Clement of Alexandria, Exhortation to the Greeks, 1
How did Joseph Smith know to restore these lost teachings which were clearly taught by the early church fathers?