Mormons believe that on May 15, 1829 John the Baptist appeared to Joseph Smith and Oliver Cowdery, and conferred the Aaronic Priesthood to them as part of
the restoration of God's Church on earth.
Because none of the Christian denominations had this and the Melchizedec Priesthood, the LDS church (Mormons call "The Church
of Jesus Christ of Latter-day Saints") claims that they all were void of authority to proclaim God's message. Mormons believe that their church is the only true church on the face of the earth.
What this means is that the foundation of the LDS Church stands or falls on two major points:
1) Their claim of a complete
apostasy of the Christian Church, and;
2) their alleged sole possession of what they call their ‘priesthood authority.'
There is no Biblical evidence of a complete falling away of the Christian Church. In fact, the Bible tells us that a complete apostasy is impossible.
For example, if the Christian Church fell into total apostasy, there would be a time when God would not be glorified in the church, right? Well, Ephesians 3:21 reads:
“Unto him be glory in the church by Jesus Christ throughout all ages, world without end.”
Since God is glorified in the church throughout all ages, a total apostasy never happened.
you can look up are Ephesians 4:11-16 which speaks
not of the Church degenerating, but maturing spiritually; Matthew 16:18,
where Jesus promised that not even the gates of Hell can prevail against His Church. Hebrews 12:27-28 and Daniel 2:44 tells us that God's Kingdom cannot be shaken and can never be destroyed.
far as the so-called ‘priesthood authority' is concerned, Hebrews chapter 7 reveals
that the Aaronic Priesthood was done away with by Jesus' death on the cross (verses 11 and 12), and that Jesus is the only One who holds the Melchizedec Priesthood, which verse 24 reveals that it cannot be transferred to another.
What all this means is that there is no Biblical evidence for starting the LDS Church on the false grounds that the Christian Church
had completely fallen away, and that the LDS Church is void of the authority of which it claims to have.