The question is: what difference does it make? What difference does it make, for example, if an under-50 Catholic today cannot distinguish between the Immaculate Conception and the virginal conception? While it is certainly better to know than not to know such information, it is hardly essential to a fully conscious Catholic life. In other areas however, religious illiteracy is more serious because it can undermine ones sense of religious identity : not only as a member of the Catholic church but more fundamentally as a Christian.
Staying with the Marian example above, it may not be a matter of overriding importance that a Catholics know the difference between the Immaculate Conception (of Mary) and the virginal conception (of Jesus); but it is surely of the highest importance that Catholics know their church does not approve, much less promote, the "worship" of Mary, as if she somehow shared in the divinity of her Son. Mary is the greatest of the saints, the most luminous model and exemplar of what Christian discipleship is all about: giving oneself totally to the following of Christ, allowing Christ and his gospel to shape and transform ones whole being, ones whole value system, ones whole network of relationships with others.
AS in many things, however, it is better to know too little than too much. Thus, in Marian devotion, there are many Catholics for whom Mary is a virtual cipher. They never advert to here, much less seek her intercession through prayer. If they belong to a parish dedicated to Mary, this is about as close as they get to any conscious devotional relating to the mother of Jesus. On the other hand, there are Catholics for whom Mary shares almost equal spiritual billing with her Son. These regard her as his key partner in the drama of salvation. She is the one who, in various appearances around the world, constantly goads us into greater fidelity to Jesus' message : using threats of damnation and global catastrophe as her primary tools of persuasion.
Such Catholics as these are constantly bemoaning the loss of religious literacy among their fellow Catholics. But one has to ask which is worse: illiteracy or a serious distortion of what one takes to be Catholic doctrine? The religiously illiterate Catholic may not know the difference between the Immaculate Conception and the virginal conception; but that same Catholic knows instinctively that if we have been redeemed, it is Jesus Christ who redeemed us, not his mother.
Such Catholics also know (again instinctively rather than consciously) that the Gospel means "Good News," not bad news, that the essence of the message that Christ came to share with us is a message of joy and hope, not gloom and doom. Is "sin" part of the total picture? Of course it is. Can sin derail our progress in the life of the Spirit? Of course it can. But in the Catholic faith tradition : based as it is on the preaching of Jesus himself and of his apostles, "where sin increased, grace overflowed all the more" (Romans 5:20). The power of God's grace is always stronger than the pull of sin. The fundamental message of the Gospel therefore is not that the world and all of us in it are teetering on the brink of damnation, but that the whole of God's creation is destined for glory (Romans 8:18-25).
Tom Beaudoin, a professor of theology at Boston College and a widely published commentator on the faith of younger Catholics like himself, distinguishes between "conceptual literacy" and "performance literacy," that is, between "knowing" what one is to believe as a Catholic and actually "doing" what is consistent with the church's core beliefs. Beaudoin regards this "performance literacy" as a major success story for post-Vatican II religious education, expressed in the practice of volunteerism, social justice, ecumenical issues and lay responsibility within the church.
In the end, it is not the one saying "Lord, Lord!" who enters the kingdom of heaven, but the one who actually does the will of God (Matthew 7:21).