It’s often supposed that Vatican I’s teaching on papal infallibility really just came out of nowhere. While the saints, synods, and popes of the first millennium certainly made lofty claims about the Roman Church, affirming her headship over the Episcopate and pristine orthodoxy, we’re assured that all of them would have stood in unison against
I'm unsure about your premise that the first millennium authorities you quote would have necessarily accepted the dogmas of Vatican 1, which was promulgated within a church that had clearly succumbed to heresy by accepting a non-canonical variation of the Nicaeano-Constantinopolitan creed in 11th century. Their arguments that 'the successors of St Peter' would always hold to pure orthodoxy had thereby obviously failed. If they had been confronted with the 11th century reversion of Rome under its German occupation, St Maximus, St Theodore, for example, would have been forced to reassess their thinking, as expressed above. Their logic about the position of the 'successors of St Peter' may therefore be rather contingent upon the record of Rome's faithful orthodoxy up until their time.
I'm unsure about your premise that the first millennium authorities you quote would have necessarily accepted the dogmas of Vatican 1, which was promulgated within a church that had clearly succumbed to heresy by accepting a non-canonical variation of the Nicaeano-Constantinopolitan creed in 11th century. Their arguments that 'the successors of St Peter' would always hold to pure orthodoxy had thereby obviously failed. If they had been confronted with the 11th century reversion of Rome under its German occupation, St Maximus, St Theodore, for example, would have been forced to reassess their thinking, as expressed above. Their logic about the position of the 'successors of St Peter' may therefore be rather contingent upon the record of Rome's faithful orthodoxy up until their time.