Pope Eugenius IV, Bull “Cantata Domino”, Council of Florence: “It firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart “into everlasting fire which was prepared for the devil and his angels” [Matt. 25:41], unless before the end of life the same have been added to the flock; and that the unity of the ecclesiastical body is so strong that only to those remaining in it are the sacraments of the Church of benefit for salvation, and do fastings, almsgiving, and other functions of piety and exercises of Christian service produce eternal reward, and that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church.”(DENZ.714)
Pope Leo XIII, Encyclical Satis Cognitum: “9. The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium. Epiphanius, Augustine, Theodore :, drew up a long list of the heresies of their times. St. Augustine notes that other heresies may spring up, to a single one of which, should any one give his assent, he is by the very fact cut off from Catholic unity. “No one who merely disbelieves in all (these heresies) can for that reason regard himself as a Catholic or call himself one. For there may be or may arise some other heresies, which are not set out in this work of ours, and, if any one holds to one single one of these he is not a Catholic” (S. Augustinus, De Haeresibus, n. 88).”
Pope Eugenius IV, Bull Exultate Deo, Council of Florence: “Holy baptism, which is the gateway to the spiritual life, holds the first place among all the sacraments; through it we are made members of Christ and of the body of the Church. And since death entered into the universe through the first man, “unless we are born of water and the Spirit, we cannot,” as the Truth says, “enter into the kingdom of heaven” (cf.John 3:5).”(DENZ.696)
Pope Pius XII, Encyclical Mystici Corporis: “22. Actually only those are to be included as members of the Church who have been baptized and profess the true faith, and who have not been so unfortunate as to separate themselves from the unity of the Body, or been excluded by legitimate authority for grave faults committed. “For in one spirit” says the Apostle, “were we all baptized into one Body, whether Jews or Gentiles, whether bond or free.” As therefore in the true Christian community there is only one Body, one Spirit, one Lord, and one Baptism, so there can be only one faith. And therefore if a man refuse to hear the Church let him be considered — so the Lord commands — as a heathen and a publican. It follows that those are divided in faith or government cannot be living in the unity of such a Body, nor can they be living the life of its one Divine Spirit.
23. Nor must one imagine that the Body of the Church, just because it bears the name of Christ, is made up during the days of its earthly pilgrimage only of members conspicuous for their holiness, or that it consists only of those whom God has predestined to eternal happiness. it is owing to the Savior’s infinite mercy that place is allowed in His Mystical Body here below for those whom, of old, He did not exclude from the banquet. For not every sin, however grave it may be, is such as of its own nature to sever a man from the Body of the Church, as does schism or heresy or apostasy. Men may lose charity and divine grace through sin, thus becoming incapable of supernatural merit, and yet not be deprived of all life if they hold fast to faith and Christian hope, and if, illumined from above, they are spurred on by the interior promptings of the Holy Spirit to salutary fear and are moved to prayer and penance for their sins.
My Comment: These quotes should be adequate to prove that one must believe in everything the Church teaches, right down to the least doctrine. One must be Baptized and profess the Catholic faith to be a member of the Church and have the title of Catholic.