What must we do to be open to Grace? There are two ways to answer this – the first seems the correct one. The effort of the whole community is involved; an atmosphere of mutual respect and trust through which we try to come to an understanding of truth and are willing to receive it – whoever it is comes to such an enlightened position. The second answer seems self-defeating. Since intellects are clouded by sin, God must have instituted a guaranteed channel – the channel Catholics adhere to is the Magisterium of the Church. If the Pope declared something to be natural law it was guaranteed to be right reason.
This second view is self-defeating because it combines two sources of morality into one. Only authority is left, for reason and common sense have been eliminated. The point of the natural law was to guarantee a place for reason to show the continuity between reason and revelation. It makes us passive in our responsibilities, leaving only an obligation to obey commands. This sort of reasoning led to the rise of Nazism.
There is something else to consider. Common sense judgments do not come out of the blue; they are formed in particular situations, from actual experience. The difference between the two is highlighted by the common reaction to Humane Vitae of Paul VI, when many understood it as infallible teaching. However, if everything is not decided by right reason, we need to ask: are all moral teachings controlled by the Church, and therefore the Church can change them; or, have some questions been decided by Jesus so that they can never be changed? For instance the issue of divorce.