The Vatican has appointed an American bishop to rein in the largest and most influential group of Catholic nuns in the United States, saying that an investigation found that the group had “serious doctrinal problems.”

The Vatican’s assessment, issued on Wednesday, said that members of the group, the Leadership Conference of Women Religious, had challenged church teaching on homosexuality and the male-only priesthood, and promoted “radical feminist themes incompatible with the Catholic faith.”

The sisters were also reprimanded for making public statements that “disagree with or challenge the bishops, who are the church’s authentic teachers of faith and morals.” During the debate over the health care overhaul in 2010, American bishops came out in opposition to the health plan, but dozens of sisters, many of whom belong to the Leadership Conference, signed a statement supporting it — support that provided crucial cover for the Obama administration in the battle over health care.

The conference is an umbrella organization of women’s religious communities, and claims 1,500 members who represent 80 percent of the Catholic sisters in the United States. It was formed in 1956 at the Vatican’s request, and answers to the Vatican, said Sister Annmarie Sanders, the group’s communications director.