A brief explanation of ecumenism showing what the Church teaches on this issue and how Vatican II contradicts it.
The description of this movement of dialogue and mutual exchange on religious questions with non-Catholics, and this on a basis of equality, is first made in a papal encyclical of Pope Pius XI, in Mortalium animos, On Fostering True Religious Unity, published in 1928.
This is the pope’s description:
Assured that there exist few men who are entirely devoid of the religious sense, they seem to ground on this belief a hope that all nations, while differing indeed in religious matters, may yet without great difficulty be brought to fraternal agreement on certain points of doctrine which will form a common basis of the spiritual life. With this object, congresses, meetings, and addresses are arranged, attended by a large concourse of hearers, where all without distinction, unbelievers of every kind as well as Christians, even those who unhappily have rejected Christ and denied His divine nature or mission, are invited to join in the discussion." (§2)
Follows immediately afterwards the pope’s condemnation of "the pan-christians", whose "fair and alluring words cloak a most grave error, subversive of the foundations of the Catholic Faith" (§3):
Such efforts can meet with no kind of approval among Catholics. They presuppose the erroneous view that all religions are more or less good and praiseworthy (this is the error of indifferentism), inasmuch as all give expression, under various forms, to that innate sense which leads men to God and to the obedient acknowledgment of His rule. Those who hold such a view are not only in error; they distort the true idea of religion, and thus reject it, falling gradually into naturalism and atheism. To favor this opinion, and to encourage such undertakings is tantamount to abandoning the religion revealed by God." (§2)
In his Instruction of the Ecumenical Movement (Instructio de Motione Oecumenica) in 1949, Pope Pius XII ordered that, in opposition to such "dangerous indifferentism":
Catholic doctrine must be propounded and explained in its totality and in its integrity. It is not permitted to pass over in silence or to veil in ambiguous terms what is comprised in the Catholic truth on the true nature and stages of justification, on the constitution of the Church, on the primacy of jurisdiction of the Roman Pontiff, on the unique true union by the return of separated Christians to the one true Church of Christ."
And yet, this is precisely what has not been done since Vatican II, in attempting to follow the contrary request not to offend the sensitivities of our "separated brethren" in the Vatican II decree "On Ecumenism" (Unitatis Redintegratio). This is how that document defines Ecumenism, with none of the precautions laid out by Pope Pius XII against indifferentism:
The term "ecumenical movement" indicates the initiatives and activities encouraged and organized …to promote Christian unity (i.e., the apparent unity, outside the truth, of different denominations or churches getting along). These are: first, every effort to avoid expressions, judgments and actions which do not represent the condition of our separated brethren with truth and fairness and so make mutual relations with them more difficult."
The document also lists as other ecumenical activities dialogue, cooperation for the common good of humanity and common prayer (U.R. §4). These activities are all based upon the belief, already condemned in advance by Pope Pius XI, that all religions are more or less good or praiseworthy, expressed in this way in the Vatican II document on Ecumenism:
Separated communities and churches as such …have been by no means deprived of significance and importance in the mystery of salvation. For the spirit of Christ has not refrained from using them as means of salvation." (U.R. §3)
Clearly this leaves no place for the defined dogma, "Outside the Church, no salvation" (Lateran IV, Denzinger 802).
Answered by Fr. Peter R. Scott