In a recent interview arranged by Fr. Antonio Spadaro, S.J., the editor of La Civiltà Cattolica, prior to the trip to Sweden for an ecumenical gathering anticipating next year’s 500th anniversary of the Reformation, Pope Francis expressed something that he has voiced several times during his pontificate: “to proselytize in the ecclesial field is a sin.” He added: “Proselytism is a sinful attitude.”
This is strong language and deserves careful attention, because many think the pope is saying the Catholic Church should no longer evangelize other Christians. That’s a large question that would take extensive treatment. Here, I’m going to limit myself to the “ecclesial field,” in the pope’s phrase, of ecumenical dialogue. Unfortunately, Francis did not define what he means by proselytizing, and did not distinguish it from evangelizing. He simply states that proselytism as such is a sin. But he doesn’t tell us why. Nor does he distinguish between unethical and ethical means of proselytizing.
It helps to turn to a document produced by a working group organized by the Catholic Church and the World Council of Churches: “The Challenge of Proselytism and the Calling to Common Witness.” The group formulated some basic points about what would constitute improper “proselytizing” in an ecumenical context:
1. Unfair criticism or caricaturing of the doctrines, beliefs, and practices of another church without attempting to understand or enter into dialogue on those issues.
2. Presenting one’s church or confession as “the true church” and its teachings as “the right faith” and the only way to salvation.
3. Portraying one’s own church as having high moral and spiritual status over against the perceived weaknesses and problems of other churches.
4. Offering humanitarian aid or educational opportunities as an inducement to join another church.
5. Using political, economic, cultural, and ethnic pressure or historical arguments to win others to one’s own church.
6. Taking advantage of lack of education or Christian instruction, which makes people vulnerable to changing their church allegiance.
7. Using physical violence or moral and psychological pressure to induce people to change their church affiliation.
8. Exploiting people’s loneliness, illness, distress or even disillusionment with their own church in order to “convert” them.
Pared down for present purposes, only point 2. raises fundamental ecclesiological questions (the rest may be accepted as unethical means, without any theological difficulty). The ecclesiological question has to do with the unity and diversity of the one Church.
In an ecumenical dialogue, we must avoid the following either/or dilemma in answering this question:
either affirming that the Church of Christ fully and totally subsists alone in its own right in the Catholic Church, because the entire fullness of the means of salvation are present in her (Lumen gentium §8; Unitatis redintegratio §§3-4; Ut Unum Sint, §14), and then implausibly denying that Orthodoxy and the historic churches of the Reformation are churches in any real sense, such that there exists an ecclesial wasteland outside the Church’s visible boundaries;
or affirming that they are churches in some sense, in a lesser or greater degree to the extent that there exists elements of truth and sanctification in them, but then accepting ecclesiological relativism or pluralism – meaning thereby that the one Church of Christ Jesus subsists in many churches, with the Catholic Church being merely one among many churches.
Clearly, the Church regards non-Catholic Christians as belonging, imperfectly, to the household of faith, i.e. the Catholic Church, and hence she speaks of them as “separated brethren,” brothers and sisters in the Lord Jesus Christ.
Indeed, the Church expresses its identity as the one Church of Christ by establishing a relationship of dialogue with these churches. Dialogue means we all learn from each other and don’t dismiss each other out of hand. We must still speak the truth, in love (Ephesians 4:15) in our search to realize a fuller unity.
“With non-Catholic Christians,” the Congregation for the Doctrine of the Faith states, “Catholics must enter into a respectful dialogue of charity and truth, a dialogue which is not only an exchange of ideas, but also of gifts, [indeed, a dialogue of love (John Paul II, Ut Unum Sint, §§28, 47)], in order that the fullness of the means of salvation can be offered to one’s partners in dialogue.
In this way, they are led to an ever deeper conversion in Christ.” (“Doctrinal Note on Some Aspects of Evangelization”) This is receptive ecumenism at its best. It is the fruit of an ecumenism of encounter and spiritual friendship.
In this ecumenical dialogue, “above all, there is  listening, as a fundamental condition for any dialogue, then,  theological discussion, in which, by seeking to understand the beliefs, traditions and convictions of others, agreement can be found, at times hidden under disagreement.”
This second dimension includes ecumenical apologetics. Such apologetics and receptive ecumenism are not at odds. It is best illustrated in a book such as Matthew Levering’s Mary’s Bodily Assumption.
Furthermore, “Inseparably united with this is another essential dimension of the ecumenical commitment:  witness and proclamation of elements which are not particular traditions or theological subtleties, but which belong rather to the Tradition of the faith itself.” This third dimension flows from the Catholic conviction that the entire fullness of the means of salvation is present in the Catholic Church.
“In this connection,” the Congregation adds, “it needs also to be recalled that if a non-Catholic Christian, for reasons of conscience and having been convinced of Catholic truth, asks to enter into the full communion of the Catholic Church, this is to be respected as the work of the Holy Spirit and as an expression of freedom of conscience and of religion. In such a case, it would not be a question of proselytism in the negative sense that has been attributed to this term.”
This is the essential difference between negative and positive proselytism, the latter – as the Church clearly teaches – being an integral aspect of evangelization, even in the “ecclesial field.”