Most of us rarely mention Protestant denominations because we rarely have any reason to do so.
One is bound to acknowledge that the Catholic Church, founded by Christ and perpetuated through apostolic succession, is not a denomination among many, but the one, holy, catholic, and apostolic Church, outside of which there is no ordinary means of salvation (cf. Extra Ecclesiam nulla salus, Lateran IV, 1215; Dominus Iesus, 2000). The term "Roman Catholic" is a misnomer when applied to the universal Church, for she is not a regional sect but the Mystical Body of Christ, governed by the Successor of Peter and possessing the fullness of truth and sacramental grace. Ecumenism, properly understood, does not imply doctrinal compromise nor parity among ecclesial communities, but rather the call to restore unity by drawing separated brethren into full communion with the Church Christ Himself established (cf. Unitatis Redintegratio, §3; Lumen Gentium, §8). The Church recognises that elements of sanctification and truth exist outside her visible structure, yet these are ordered towards Catholic unity and derive their efficacy from her own plenitude (cf. Catechism of the Catholic Church, §819).
One ought not be deceived by sentimental appeals to “diversity” that obscure the gravity of schism and heresy. The Second Vatican Council, while initiating a pastoral openness to dialogue, did not revoke the dogmatic claims of the Church nor dilute her divine constitution. Ecumenism is grounded in Christ’s prayer “that they may all be one” (John 17:21), but this unity is not a vague spiritual fellowship—it is visible, hierarchical, and sacramental, centred on the Eucharist and governed by the Petrine office (cf. Ut Unum Sint, §20–23). The Church does not seek mutual recognition of doctrinal error, but conversion to the truth. Protestant communities, though often sincere, lack valid sacraments (save baptism) and apostolic succession, and thus cannot be considered churches in the proper sense (cf. Dominus Iesus, §17). Thou art called, therefore, not to relativise truth, but to labour for the return of all Christians to the one fold under the one Shepherd.