The above question has been sitting in my mind for a while now, especially since my research into dissident "Catholic" groups (e.g. OECTA, Catholics for Choice, Wymynpriest, CNWE, etc.). How could people who once were in possession of the Truth - abandon it? One answer could be they never had it. The Catholic Faith is an infused virtue from God; it may well be that many of these people were just social Catholics to begin with. But another answer becomes possible. It may well that it was lost through be some crisis of ontology, or being.
The Corruption of Truth: Modernism
The Modernist thinker develops and follows the agnostic Kantian-Hegelian path - one of subjectivism, and truth being a never ending product of the dynamic mind. The logical extension of Kant was that religion is a product of the mind, the mind as found within a given moment in history, within a given culture. Truth is not absolute, it is not outside of the mind to be conformed to by the reasoning person. Rather, it is something personal and moulded in the mind. Religion, faith, religious experience is "true" for the "believer", but can be false for the next "believer". What I feel is true - for me. What you feel is true - for you. And so on. Reality, for the Modernist is from within, the object of religion is conformed to the mind. Faith comes from within. Thus, it was no coincidence that Pius X (Pasciendi) wrote that one logical consequence of Modernism was that to be consistent not only was Catholicism true, but so was Islam and so on.
Religious experience is immanent in the mind of the person. With the reduction of religious experience to subjective feelings, religion is reduced to external ritual and moves towards agnosticism. Once religion is no longer objective reality, but subjective thought, all religions become equally true (and hence, equally false). The result is that Catholicism is uprooted, and religious "insanity" (c.f. Mirari Vos, Gregory XVI) abounds.
"And I, if I be lifted up, will draw all men unto myself (John 12:32)
Our Lord Jesus Christ outlined his whole programme for the history of humanity. He drew to its fruition the Theology of History as proclaimed by Abraham and the Prophets, and - with the Holy Spirit operating through St. John the Baptist - fulfilled the Will of His Father by revealing His Sacred and Divine Identity and Mission to His people, the Jewish nation.
However, now, the call was universal - to all men. For example, the Roman centurion prayer to Christ for mercy on his ill servant ("Domine, non sum dignus"...). This universality of Salvation - more urgent than ever in a world converging on a world culture - must be therefore preached with clarity to all men without exception - to Jew and Gentile alike. Such preaching must be bound with love, but true love supposes truth. The Church temporal, being in possession of the absolute truth in faith and morals is bound before Her Lord and Master, Christ, to preach to all men (c.f. Leo XIII, Satis cognitum, Leo XIII; Quas Primas, Pius IX).
"Christ loved the Church, and delivered Himself up for it" (Eph. v., 25)
Pope Leo XIII's Satis Cognitum draws our attention to Christ's redemptive power that operates through and for the Church. Men are redeemed by being part of the Church. Thus, all men are called to the one arc of Salvation. To believe in Christ is to believe in the Church, to believe in the Church is to believe in Christ. Pope Leo writes: "But the mission of Christ is to save that which had perished: that is to say, not some nations or peoples, but the whole human race, without distinction of time or place. "The Son of Man came that the world might be saved by Him" (John iii., 17)". Man is saved by Christ, through the Church (c.f. Crossing the Threshold of Hope, John Paul II).
Post -Sixties Ecumenical Confusion leads to the abandonment of Faith and Truth
Theologically speaking only Catholics have "Faith". To speak of "faiths" is to show that one does not know what one is talking about. There is one Faith - for God cannot contradict Himself - but there are many religions. Islam, is a false religion, it is not a faith. Judaism too, though once the truth Faith, when severed from its culminating point - Our Blessed Lord - ceased to be the true Faith. Granted, Judaism, as a pre-cursor to Catholicism contains all the great truths of the Old Testament - but it is, so to speak frozen. The truths of Judaism come alive and are fulfilled in Catholicism.
The Holy Spirit, as the Church Fathers taught, acts beyond the Mystical Body to not santify men through other religions, but to impell them to unity with the Church, which has one body and one soul. A man is saved by Christ through the Church. Indeed the Holy Spirit offers gifts to men outside the Church seeking to draw them into the Church. St. Augustine is very clear that false religions do not have the Holy Ghost. A false religion cannot therefore offer a man sanctification. Sanctification is offered only through the Church by Christ. Christ is clear on this "And I, if I be lifted up, will draw all men unto Myself (John, 12:32). The Second Vatican Council - in a very contentiously fought out pastoral (hence, not dogmatic) document on Ecumenism, pointed out that any sanctification outside of the Church comes from the Church. Even the various Orthodox churches draw their dynamic from the Church to ontologically retain valid sacraments (e.g. Holy Orders). Pope Paul even had to intervene with last minute changes to ensure orthodoxy (c.f. Second Vatican Council, Abbott edition footnotes).
Following the Council, Pope Paul warned Catholics (1976) that they were two grave dangers that pressed on Catholicism: protestantism (with its individualized "faith", and rejection of the magisterium), and Marxism (with the horizontalization and sociological reduction of religion to "this world" social justice movements). Earlier, in 1968, he had defined these same two evils of individualism and worldly messianism as: atheism and sociological Christianity. We can see these two trends permeating the various dissident sects in varying degrees. Taking up individualized pick-and-choose religion (protestantism, which leads through agnosticism to atheism), focusing on saving the world rather than one's soul, and abandoning Christ's Church through disobedience, brought the logical consequence of abandoning the Catholic Faith (even if one went to church every Sunday).
To conclude with the words of Pius XII (Mystici corporis):
We have committed to the protection and guidance of heaven those who do not belong to the visible Body of the Catholic Church...even though by an unconscious desire and longing they have a certain relationship with the Mystical Body of the Redeemer, they still remain deprived of those many heavenly gifts and helps which can only be enjoyed in the Catholic Church. Therefore may they enter into Catholic unity and, joined with Us in the one, organic Body of Jesus Christ, may they together with us run on to the one Head in the Society of glorious love. Persevering in prayer to the Spirit of love and truth, We wait for them with open and outstretched arms to come not to a stranger's house, but to their own, their father's home.