It appears he did take an oath of silence when forced to abdicate the Seat.
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An oath whose purpose is to destroy the Church would obviously not be binding. In fact, it would be a sin to observe it. This is not only common sense, it is something explicitly taught to children in 1st Communion catechism class. To say that a cardinal did not know this is absurd.
This is probably the most difficult angle of the Siri thesis. I don't know of any version of it in which Siri doesn't come out as the worst coward since Pontius Pilate. At least Pontius Pilate could claim he was a simple pagan and didn't know that Christ was God, and was threatened with his life. But Siri was threatened as a cardinal of the Catholic Church and caved in, -- as pope, no less, the vicar of Christ, with the most powerful graces of state of any human being on this planet -- and consented to hand over the reins of the Church to its worst enemies, who were psychopathic mass murderers (if you believe the hydrogen-bomb-over-Rome theory), and allowed them to take over the Church?! A cardinal, who not only had (presumably) the virtue of Faith, and should have believed that God would triumph over His enemies, but also had almost 2,000 years of history behind him to see that the Church had overcome countless such attacks and persecutions and would easily triumph over this one again? How could he think it would be a good idea to allow people threatening mass murder and/or schism to take the papacy from him?
On a certain level, I don't think it matters much whether he or Roncalli won that election. I think either one was just as bad.
And if I had to choose to be in the eternal shoes of either Pontius Pilate or Cardinal Siri, I'd choose Pontius Pilate, since he was less responsible for his actions.