“For it has never been allowed that that be discussed again which has once been decided by the Apostolic See” St. Boniface I, Letter to Rufus, Bishop of Thessaly 422
“The Apostolic See’s confession of faith is unassailable; it is impossible for it to be stained by any false doctrine or be contaminated by any error.” St. Gelasius I, Letter to the Emperor Anastasius 494
“The Roman See has never erred, and never will err, because of Christ’s promise,” St. Agatho, Letter to the Synod at Constantinople 680
“If certain persons should be disobedient unto the words spoken by Him through us, let them understand that they will entangle themselves in no slight transgression and danger.” St. Clement, Letter to the Corinthians 95
How powerful are these statements. It is good to recall them with confidence. We cannot just get rid of Vatican II or anything that the Popes are saying. Even when not speaking infallibly their speech should be highly regarded.
I got these quotes from available sources online or from The Wisdom of the Popes: A Collection of Statements by Thomas J. Craughwell which is an excellent (though expensive) book I was able to find at a book fair. I highly recommend it. It goes through several virtues, social issues and historical circumstances by giving quotes from/about Popes. It has served me well. This posts is another example.