First of all, Historic Christianity -- which can be traced directly to the Apostles -- is unambiguously Catholic. Protestantism is an aberration that originated in the 16th Century from a cobbling together of secular humanist ideas, Nominalist Philosophy, and fringe elements from the anti-realistic, voluntaristic, and anti-intellectual trends that were current in late Medieval urban centers.
Secondly, it is the Protestants who abandoned the faith delivered one and for all to the saints for "another gospel". It must be made clear that it was the Protestants who broke fellowship with us Catholics to go off and invent novel cults in discontinuity with the historic Churches of East and West. We have not changed our doctrine or our ecclesiastical affiliation. We catholics are in communion with the Church in all times and places.
Thirdly, the Catholic Church is not just a collection of individual believers. We are the Mystical Body of Christ and each of us is a functioning organ in that body. We are incorporated into Christ by Baptism which makes us a new creation and joins us to Christ in his life death and resurrection (Romans 6).
Fourthly, membership in the Catholic Church is necessary for salvation. This has been stated numerous times from the Bible (Acts 4:12) and Tradition (Unam Sanctam). No one can be saved apart from the Catholic Church.
The Second Vatican Council summed it up this way in Lumen Gentium :
So there is no question about it. The Catholic Church considers herself to be the one true Church founded by Jesus and teaches that membership in her is the norm of salvation. Anything other than full communion with the Catholic Church is less than adequate.
14. This Sacred Council wishes to turn its attention firstly to the
Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism(124) and thereby affirmed also the necessity of the Church, for through baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved.
They are fully incorporated in the society of the Church who, possessing the Spirit of Christ accept her entire system and all the means of salvation given to her, and are united with her as part of her visible bodily structure and through her with Christ, who rules her through the Supreme Pontiff and the bishops. The bonds which bind men to the Church in a visible way are profession of faith, the sacraments, and ecclesiastical government and communion. He is not saved, however, who, though part of the body of the Church, does not persevere in charity. He remains indeed in the bosom of the Church, but, as it were, only in a "bodily" manner and not "in his heart."(12*) All the Church's children should remember that their exalted status is to be attributed not to their own merits but to the special grace of Christ. If they fail moreover to respond to that grace in thought, word and deed, not only shall they not be saved but they will be the more severely judged.(13*)
Catechumens who, moved by the Holy Spirit, seek with explicit intention to be incorporated into the Church are by that very intention joined with her. With love and solicitude Mother Church already embraces them as her own.
124 Cf. Mc 16, 16; Jn. 3, 5.
(12*) Cfr. S. Augustinus, Bapt. c. Donat. V, 28, 39; PL 43, 197: Certe manifestum est, id quod dicitur, in Ecdesia intus et foris, in corde, non in corpore cogitandum. Cfr. ib., III, 19, 26: col. 152; V, 18, 24: col. 189; In Io. Tr. 61, 2: PL 35, 1800, et alibi saepe.
(13*) Cfr. Lc. 12, 48: Omni autem, cui multum datum est, multum quaeretur ab eo. Cfr. etiam Mt. 5, 19-20; 7, 21-22; 25 41-46; Iac., 2, 14.
But Lumen Gentium goes on to say:
15. The Church recognizes that in many ways she is linked with those who, being baptized, are honored with the name of Christian, though they do not profess the faith in its entirety or do not preserve unity of communion with the successor of Peter. (14*) For there are many who honor Sacred Scripture, taking it as a norm of belief and a pattern of life, and who show a sincere zeal. They lovingly believe in God the Father Almighty and in Christ, the Son of God and Saviour. (15*) They are consecrated by baptism, in which they are united with Christ. They also recognize and accept other sacraments within their own Churches or ecclesiastical communities. Many of them rejoice in the episcopate,
celebrate the Holy Eucharist and cultivate devotion toward the Virgin Mother of God.(16*) They also share with us in prayer and other spiritual benefits. Likewise we can say that in some real way they are joined with us in the Holy Spirit, for to them too He gives His gifts and graces whereby He is operative among them with His sanctifying power. Some indeed He has strengthened to the extent of the shedding of their blood. In all of Christ's disciples the Spirit arouses the desire to be peacefully united, in the manner determined by Christ, as one flock under one shepherd, and He prompts them to pursue this end. (17*)
Mother Church never ceases to pray, hope and work that this may come about. She exhorts her children to purification and renewal so that the sign of Christ may shine more brightly over the face of the earth.
(14) Cfr. Leo XIII, Epist. Apost. Praeclara gratulationis, 20 iun. 1894;
AAS 26 (1893-94) p. 707.
(15) Cfr. Leo XIII, Epist. Encycl. Satis cognitum, 29 iun. 1896: ASS 28
(1895-96) p. 738. Epist. Encycl. Caritatis studium, 25 iul. 1898: ASS 31
(1898-99) p. 11. Pius XII, Nuntius radioph. Nell'alba, 24 dec. 1941: AAS 34
(1942) p. 21.
(16) Cfr. Pius XI, Litt. Encycl. Rerum Orientalium, 8 sept. 1928: AAS
20 (1928) p. 287. Pius XII, Litt. Encycl Orientalis Ecclesiae, 9 apr. 1944: AAS
36 (1944) p. 137
(17) Cfr. Inst. S.S.C.S. Officii 20 dec. 1949: AAS 42 (1950)
The above teaching makes it clear that while we Catholics honor those who profess to be Christian, we do not find the current divisions either acceptable or permanent. The goal for us is Christian unity IN the Catholic Church, not with other religious groups outside of catholic unity.
So while we wish to be respectful of people from the Protestant religions, we cannot condone their religions since they are in both doctrinal and ecclesiastical error. Frankly, those professing any non-Catholic religion are endangering their souls. No amount of personal piety or private biblical interpretation can excuse this.
Make no mistake about it. Antipathy to the Catholic Church is a sign of non-Election. Hatred of the Body of Christ is hatred of Christ himself (Matt 25:31ff). Ignoring the ordained ministers of Christ is ignoring Christ and thereby ignoring God as well (John 13:20).
When Protestants come to argue their petty little theories by which they seek to make void the word of God and follow the teachings of mere men, they are not on a level playing field with the Catholic Magisterium. Protestants can only give their own private opinions. The Catholic Magisterium speaks under the superintendence of the Holy Spirit (Matt 10:20).
In summary, the 16th Century Protestants apostatized and left the Catholic Church and have invented thousands of separate cults which not only contradict the Catholic Church but each other on serious points of doctrine. The Catholic Church has not changed any of her teachings since before there were Protestants. Consequently it is the Protestants who must now admit that we who have remained in the Catholic Church are true Christians and that we are entitled to our own interpretation of Scripture.
Conversely, it is not possible for Catholics to recognize any Protestant group as being on par with the Catholic Church. There will always be something deficient in them. Whatever partial goods they may have, the total package of true Christianity only subsists in Catholicism.