In Mystici Corporis Christi, Pius XII:
from a heart overflowing with love We ask each and every one of them [i.e., those who are not members of the Church] to correspond to the interior movements of grace, and to seek to withdraw from that state in which they cannot be sure of their salvation.[196] For even though by an unconscious desire and longing they have a certain relationship with the Mystical Body of the Redeemer, they still remain deprived of those many heavenly gifts and helps which can only be enjoyed in the Catholic Church.
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Curious that the pope 1) describes them as in a state of uncertain salvation rather than certain reprobation and 2) having a "certain relationship" with the Church.
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I think that explicit faith (even if never externalized) in the Incarnation and Trinity is probably necessary for salvation, and I don't get the impression Pius XII is referring to infidels in the passage above. But as far as membership in the Church goes, theologians since Bellarmine have distinguished between membership
in re and membership
in voto. Pius XII appears to endorse that distinction here in his encyclical, or if not endorse it at least subtly encourage it. Compare this paragraph to the paragraph earlier in the encyclical, where the pope uses the Latin
reapse to describe those who are actually (in re) members.
Actually [reapse] only those are to be included as members of the Church who have been baptized and profess the true faith, and who have not been so unfortunate as to separate themselves from the unity of the Body, or been excluded by legitimate authority for grave faults committed.
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The word "actually" is not being used as a corrective or a contrary, it is a philosophical designation to distinguish those who are members
in re, as later he will discuss those who are members
in voto.
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Those who regard encyclicals as being capable of holding all kinds of error are likely to find nothing at all significant here, but for those who hold (correctly) that encyclicals cannot teach dangerous error, this is a passage of significance and weight that does not get discussed enough.