[Saint] Basil knows that love consists in keeping the commandments. for this reason, the spark of love, which has been put into us by the Creator, means this: “We have received interiorly beforehand the capacity and disposition for observing all divine commandments … These are not something imposed from without.” Referring everything back to its simple core, Augustine adds, “We could never judge that one thing is better than another, if a basic understanding of the good had not already been instilled in us.”
One can comprehend the primacy of the pope and its correlation to Christian conscience only in this connection. The true sense of the teaching authority of the pope consists in his being the advocate of the Christian memory. The pope does not impose from without. Rather, he elucidates the Christian memory and defends it. For this reason the toast to conscience indeed must precede the toast to the pope, because without conscience there would not be a papacy. All power that the papacy has is power of conscience. It is service to the double memory on which the faith is based—and which again and again must be purified, expanded, and defended against the destruction of memory that is threatened by a subjectivity forgetful of its own foundation, as well as by the pressures of social and cultural conformity.
—from On Conscience, a collection of two essays by Joseph Cardinal Ratzinger (Pope Benedict XVII) – from the first essay Conscience and Truth which was the keynote address of the Tenth Bishops’ Workshop of the National Catholic Bioethics Center, on “Catholic Conscience: Foundation and Formation,” February 1991
For Newman, conscience is not the same as one’s personal opinions about essential Catholic beliefs. Indeed, in his 1870 letter, he warned against “counterfeit” forms of conscience that “boast of being above all religions and to be the impartial critic of each of them.” Instead, he offered this definition: “[Conscience] is a messenger from [God], who, both in nature and in grace, speaks to us behind a veil, and teaches and rules us by His representatives. Conscience is the aboriginal Vicar of Christ.”
For a Catholic, conscience and essential Church teaching should operate together. As such, the conscience of a Catholic can never require a person to do what is wrong, contrary to the faith, or impossible.
In short, Newman’s thought reduced to a bumper sticker should read: “Conscience first; the Pope second — in everything that’s not his proper business.”