Taken directly from the documents of the First Vatican Council (from www.papalencyclicals.net/Councils/ecum20.htm):
“For the holy Spirit was promised to the successors of Peter
not so that they might, by his revelation, make known some new doctrine,
but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
Indeed, their apostolic teaching was
embraced by all the venerable fathers and
reverenced and followed by all the holy orthodox doctors,
for they knew very well that this see of St. Peter always remains unblemished by any error, in accordance with the divine promise of our Lord and Saviour to the prince of his disciples: I have prayed for you that your faith may not fail; and when you have turned again, strengthen your brethren  .
This gift of truth and never-failing faith was therefore divinely conferred on Peter and his successors in this see so that they might discharge their exalted office for the salvation of all, and so that the whole flock of Christ might be kept away by them from the poisonous food of error and be nourished with the sustenance of heavenly doctrine. Thus the tendency to schism is removed and the whole church is preserved in unity, and, resting on its foundation, can stand firm against the gates of hell.
Your move, Walter.