JimG, that the certain judgment of conscience could err is the teaching of Joseph Ratzinger. It involves a disturbance of anemnesis. What is the argument about? I am not entirely sure, really, although it appears to concern legalism, an obvious question relative to the OP as well as Pope Francis, most recently voiced in this remarks on Saturday. There is an indirect question as to the possibility of spirituality for a person.
Sorry if I misunderstood. I thought you were stating this as the teaching of Cardinal Ratzinger and of the Church and your own belief as well.
Still, it is stated in the Catechism that “conscience can make either a right judgment in accordance with reason and the divine law, or on the contrary, an erroneous judgment that departs from them. (#1786)
I don’t dispute that a man must always obey the certain judgment of his conscience (#1790).
One must follow even an erroneous conscience, but man has a duty to properly inform his conscience.
Perhaps the difficulty is in the phrase “Man has in his heart a law inscribed by God.”
What does it mean to have in the human heart a law inscribed by God? Humans are composed of body and soul. The faculties of the soul are intellect and will. A judgment of conscience is an act of the intellect, which produces judgments.
I don’t think that the entire content of the divine law could be inscribed in the intellect, since the human intellect is finite. Both the intellect and the will are formed in the image of God, the intellect seeking truth, the will seeking the good. That divine formation of the human will to seek the good is indeed innate, even though weakened by original sin.
So, though humans are designed—imprinted—to seek the good and the true, even with that divine imprinting they are, because of darkening of the intellect and a weakened will, able to choose the false and the bad.
That’s my take on the meaning of God’s law inscribed in the human heart, but I suppose that volumes could be written about it. I don’t think that it means the conscience can never err, because even though the divine law is imprinted, it must be acted upon by a human intellect and a human will, which are fallible.
It’s not a matter of either moral legalism, or moral laxity. It’s informing the intellect to know the moral law, and strengthening the will to act on it, with the help of divine grace.