A group of Catholics opposed to the sale of Caritas Christi Health Care to the private equity firm Cerberus Capital Management sent a letter to Pope Benedict XVI yesterday asking him to “call for the cessation of all negotiations with Cerberus.’’ Cerberus and Caritas representatives have said they intend to maintain the religious identity of Boston’s Catholic healthcare chain. But a stewardship agreement they negotiated with the Archdiocese of Boston would allow the hospital chain’s new owners to terminate its religious affiliation, if it became materially burdensome, in exchange for a $25 million donation to a charity chosen by the archdiocese. The Catholic church requires affiliated healthcare organizations to follow Catholic teachings, which prohibit abortion, sterilization, and euthanasia, among other procedures.