The Latter-day Saint concept of priesthood keys is not well documented in the Bible. A single passage has Jesus Christ promising to give Peter "the keys of the kingdom of heaven" (Matthew 16:19), while the book of Revelation indicates that Christ held "the keys of hell and of death" and "the key of David" (Revelation 1:18; 3:7; see Isaiah 22:22). There is no mention of keys in connection with John the Baptist.
Nevertheless, when John came to ordain Joseph Smith and Oliver Cowdery to the Aaronic Priesthood on 15 May 1829, he indicated that this priesthood "holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins" (D&C 13:1). A Christian document from Ethiopia, thought to date to the fifteenth century but containing older concepts, speaks of "John the Baptist unto whom was given the key of baptism."1
Another document of related interest comes from the Mandaeans, who claim to be descendants of the disciples of John the Baptist, whom they call by his Arabic name Yahya. Called Haran Gawaitha, the text says of John that "when he was seven years old, [the angel] Anush >Uthra came and wrote the ABC (a ba ga) for him, until when he was twenty-two years old, he had learnt all the priestly-craft (naßirutha)."2 The idea that at a young age John the Baptist was visited by an angel who delivered priesthood training is in general agreement with D&C 84:28, where we read that John "was baptized while he was yet in his childhood, and was ordained by the angel of God at the time he was eight days old." Contributed by John A. Tvedtnes