In his celebrated book, De Ecclesia Christi, the renowned
anti-Modernist Cardinal, Louis Billot, S.J, explains that the adhesion of the
Church to a Pope is an “infallible sign” of his legitimacy:
"Finally,
whatever you still think about the possibility or impossibility of the
aforementioned hypothesis [of a Pope falling into heresy], at least one point must be considered absolutely
incontrovertible and placed firmly above any doubt whatever: the adhesion of
the universal Church will be always, in itself, an infallible sign of the
legitimacy of a determined Pontiff, and therefore also of the existence of all
the conditions required for legitimacy itself. It is not necessary to look
far for the proof of this, but we find it immediately in the promise and the
infallible providence of Christ: ‘The gates of hell shall not prevail against
it,’ and ‘Behold I shall be with you all days.’ (…) As will become even more clear by what we
shall say later, God can permit that at
times a vacancy in the Apostolic See be prolonged for a long time. He can also
permit that doubt arise about the legitimacy of this or that election. He
cannot however permit that the whole Church accept as Pontiff him who is not so
truly and legitimately."
"Therefore,
from the moment in which the Pope is accepted by the Church and united to her
as the head to the body, it is no longer permitted to raise doubts about a
possible vice of election or a possible lack of any condition whatsoever
necessary for legitimacy. For the
aforementioned adhesion of the Church heals in the root all fault in the election
and proves infallibly the existence of all the required conditions."
Notice that the adhesion of the Church “heals in the roof all fault in
the election.” St. Alphonsus Ligouri, Doctor of the Church, teaches the same:
"It is of
no importance that in past centuries some Pontiff was illegitimately elected or
took possession of the Pontificate by fraud; it is enough that he was accepted
afterwards by the whole Church as Pope, since by such acceptance he would have
become the true Pontiff."
Cardinal Billot explains that the infallible providence of God will
never permit the entire Church to adhere to a false head:
"[T]he
infallible providence of God will prevent it from ever happening that the whole
Church adhere to a false head; consequently, no one will ever be accepted as
supreme pontiff who does not meet all the conditions necessary to be a member,
whatever those conditions may be. That visibility, therefore, by which the true
Church is recognizable as such, is in no way imperiled."