This article, as many of my posts, is directed to clarifying things for Catholics rather than defending or explaining them as to a non-Catholic audience. (I’ll get a page for that task in the coming months.)
In the past fifty years, the Church has, when describing her unique character among Christian groups, described herself as entrusted with “the fulness of truth.” What does this expression mean, what does it not mean, and is it the full truth?
It should be noted, by way of beginning, that the expression regards a claim about what is the case objectively. It does not regard an evaluation about the subjective condition of any individual.
What the expression obviously does not mean is that the Catholic Church has all knowledge. It would be absurd to suggest that she did. She is given no privilege to know, for instance, quantum mechanics or gravitational attraction or economical intricacies. She is given competence only the so-called “Deposit of Faith” and those matters necessarily connected with it. Now, the “Deposit of Faith” is what God reveals.
So, what the expression means is that God has entrusted to the Catholic Church all Revealed Truth, i.e., faith’s knowledge of every truth supernaturally revealed for our salvation. Now, if some Christian church or community has broken away from the Catholic Church, it of course takes with it knowledge of many of these revealed truths. However, it cannot possibly take with it all truths. (It must at least reject one key truth. I shall return to that in a moment.) Thus, different Christian churches and communities enjoy, to different degrees, a share in that fulness. The phrase “fulness of truth” in this sense sets the Catholic Church off from other Christian churches, without denying but implicitly affirming that they enjoy some share in that fulness.
Finally, is the expression sufficient? It is not. That is, it does not of itself convey the fulness of truth regarding this fulness. Let me explain.
The phrase “fulness of truth” indicates Revealed Truth as a set of truths. It thus implies that various groups can have various degrees of this fulness. In this respect, the phrase is very helpful ecumenically. For it allows the Catholic Church to affirm a truth, namely, that non-Catholic churches have various shares in this fulness.
However, Revelation is not divinely deposited as a simple “set” of truths to whatever group will accept it. Rather, it is entrusted to a guardian. That is, in bestowing revelation on his disciples, Jesus did not just scatter a bunch of truths, hoping that one of his followers might gather them all together. Rather, during his earthly ministry he immediately and directly constituted a Church to guard the deposit in its integrity. But he constituted only one Church (unam sanctam catholicam et apostolicam ecclesiam). This Church he constituted as a visible hierarchical society, whose hierarchical character is of divine right and not of human invention. (Nor, we should add, can any authority whatsoever alter this hierarchical structure, which consists in the bishops in communion with the pope.) This Church he deputed to be the bearer of the way of life he inaugurated, the religion he established.
Now, in order to proclaim this truth – that Christ established the Catholic Church to bear his religion – there is need for another phrase. That phrase is one true religion. The Catholic Church not only has the fulness of revelation but is the one true religion.
Now, this phrase “one true religion” cuts a little deeper than the phrase “fulness of truth”. For, once again, “fulness of truth” indicates the Deposit of Faith as a kind of “set of truths” that different groups might appreciate to different degrees. (The scholastics would call this an expression taken from a “material consideration”. We might liken this somewhat to a list of ingredients. The ingredients don’t give the unity of the whole – the form does that – but they are elemental in that whole. There is so much sugar, so much flour, etc., in the cake.)
But the phrase “one true religion” indicates something deeper, namely, that there is one authority which is to bear responsibility for the one religion Jesus established. (The scholastics would call this an expression taken from a “formal consideration”. Such a consideration looks at the thing as a whole. For instance, what is a “cake”; where is “the cake”? You don’t really have a cake unless you have not only all the ingredients but the right mixture baked at the right temperature for the right time. The net result is the real cake.) Now, here’s the rub: If there is only one such authority, only one Church actually established by Christ, then all other claims to be the Church established by Christ are false. All other religions are invalid. All other claimants to the authority Christ gave his Church are invalid.
Thus, all religions but the Catholic religion are, formally considered, false. They may have many “elements of truth” in them, but they do not bear the deputation of God. All their “elements of truth and sanctification” are calling out not merely for “more truths”. They are calling out for that. But more importantly, they are calling out for that final wholeness of being the true Church deputed by Christ that is either present or not present. This final wholeness is not merely one more truth but – as one more ingredient – but an alteration of form! It is integration into the Catholic Church. Without this integration, the search for the fulness is painstaking and endless. With this integration, that fulness is enjoyed at once, and in its proper home.
Now, there is a key implication for ecumenism with these considerations. The Catholic Church is irrevocably committed to ecumenism – the endeavor to achieve full and visible unity among all Christian churches and communities. Regarding the phrase “fulness of truth,” the Catholic Church strives that other churches might enjoy each element of truth which she enjoys. In this regard, she seeks that these others as it were become like her. But with regard to the phrase “one true religion,” the Catholic Church cannot but strive to integrate into her own structure any Christian community or church which is non-Catholic.
Thus, there is need for both phrases. The first – “fulness of truth” – allows the nuance of different degrees of participation in the set of truths divinely revealed. The second – “one true religion” – allows one to state the crucial truth of integrity, of existence as the one true Church deputed by Christ to worship God. Note that this expression has not entirely disappeared in the last fifty years. (See Dignitatis humanae, art. 1.) However, it has unfortunately been lost to sight. I say unfortunately, because failure to convey this is failure to teach the fulness of truth.