“And no man taketh this honor unto himself, but he that is called of God as was Aaron” (Hebrews 5:4).
How was Aaron called? By his brother Moses, the prophet, who then laid hands on his head and conferred the priesthood upon Aaron. Where did Moses get the authority? From Jethro, his father-in-law, a Semite, who held the Melchizedec priesthood (D&C 84:6-7).
And where do modern-day Christian ministers and leaders derive their priesthood authority? From schools, universities, and institutes. From a calling they feel deeply and sincerely. I do not question the sincerity of another’s belief, but God explained clearly the process of getting the priesthood, it must come from one who already has it.