From the Catholic Encyclopedia:
To the Catholic the distinction of Church and sect presents no difficulty. For him, any Christian denomination which has set itself up independently of his own Church is a sect. According to Catholic teaching any Christians who, banded together refuse to accept the entire doctrine or to acknowledge the supreme authority of the Catholic Church, constitute merely a religious party under human unauthorized leadership. The Catholic Church alone is that universal society instituted by Jesus Christ which has a rightful claim to the allegiance of all men, although in fact, this allegiance is withheld by many because of ignorance and the abuse of free-will. She is the sole custodian of the complete teaching of Jesus Christ which must be accepted in its entirety by all mankind. Her members do not constitute a sect nor will they consent to be known as such, because they do not belong to a party called into existence by a human leader, or to a school of thought sworn to the dictates of a mortal master. They form part of a Church which embraces all space and in a certain sense both time and eternity, since it is militant, suffering, and triumphant. This claim that the Catholic religion is the only genuine form of Christianity may startle some by its exclusiveness. But the truth is necessarily exclusive; it must exclude error just as necessarily as light is incompatible with darkness. As all non-Catholic denominations reject some truth or truths taught by Christ, or repudiate the authority instituted by him in his Church, they have in some essential point sacrificed his doctrine to human learning or his authority to self-constituted leadership. That the Church should refuse to acknowledge such religious societies as organizations, like herself, of Divine origin and authority is the only logical course open to her. No fair-minded person will be offended at this if it be remembered that faithfulness to its Divine mission enforces this uncompromising attitude on the ecclesiastical authority. It is but a practical assertion of the principle that Divinely revealed truth cannot and must not be sacrificed to human objection and speculation. But while the Church condemns the errors of non-Catholics, she teaches the practice of justice and charity towards their persons, repudiates the use of violence and compulsion to effect their conversion and is ever ready to welcome back into the fold persons who have strayed from the path of truth...It is not in the further rejection of truth that the divisions of Christianity can be healed, but in the sincere acceptance of what has been discarded; the remedy lies in the return of all dissenters to the Catholic Church.
As I said previously, make no mistake. The RCC does not consider Protestants a reasonable alternative, but a heretical sect of dissenters that must be brought back under the authority of Rome.