Ok, we have laid the laborious groundwork for assessing de Lubac’s thesis. How much labor this has required! How much thought! Not mere emotive guesswork. We are seeking the truth, not simply our (unformed, often ill-begotten) feelings about it. Let us now examine his thesis.
De Lubac says that we as human have but one meaningful end, beatific union with God. More precisely, he said that human nature as such has but one meaningful end, beatific union with God.
The theologians at the time were up in arms over this statement. Why? Because they realized that we need grace in order to attain vision. But whatever is required in a thing that it be able to attain its meaningful end is a (dynamical) debitum naturae. Ergo, they concluded, on de Lubac’s supposition, grace must be a debitum naturae.
In point of fact, de Lubac sometimes implies that grace is just such a required thing. Let us read his own words:
“As a result—at least so it seems—how could the just and good God frustrate me, if it were not I who by my own fault turned myself away from him freely?”
De Lubac here implies that grace is a debitum naturae. Why? Because debitum pertains to justice. The rhetorical question “How could the just God frustrate me” implies that God in justice could not frustrate me, unless sin be brought into the picture. But remember, we are thinking precisely and scientifically here. We must abstract from all our personal traumas, desires, emotions, etc., in order in a manly way to think this through. We are considering human nature as such.
Now, if – abstracting from sin – God could not in justice frustrate me with regard to X, then the things requisite for X are debita naturae. De Lubac therefore asserts that grace, which is necessary for beatific union with God, is a debitum naturae. But to hold that grace is a debitum naturae is to contradict the faith of Holy Mother Church. This is why so many theologians criticized de Lubac, and severely.
Pius XII, of immortal memory, in his must-read encyclical, Humani generis, art. 26, (1950), had this to say:
“Others destroy the true ‘gratuity’ of the supernatural order when they say that God is not able to establish beings gifted with intellect without ordering and calling them to the beatific vision.” (DS 3018).
 Le mystère, p. 80. See also “Le mystère du surnaturel,” 91.