Authority

Authority is a huge concept in Mormonism. It is connected exclusively with the LDS priesthood. Thus, the only valid voices in religious matters are LDS authorities; the only valid religious rites are those performed by members of the LDS priesthood. This is why they consider only the baptisms performed by a priesthood holder as genuine. 

Biblical Christianity teaches that all believers are priests before God (1 Peter 2:9). There is now no special priesthood with special authority.