"Unless I am convinced by the testimonies of Scripture or evident reason (for I do not believe either Pope or councils alone, since it is certain that they have both erred frequently and contradicted themselves)...I neither can nor wish to revoke anything." (This was said by Martin Luther at Worms in 1521 while still a Catholic priest).
"No enlightened Catholic holds the pope's infallibility to be an article of faith. I do not; and none of my brethren, that I know of do." (This was said by Bishop John Purcell in the Campbell-Purcell Debate on the Roman Catholic Religion in 1837. The Debate was later printed in a book and Bishop's Purcell's statement is found on page 27. He made his remark before papal infallibility was decreed by the Vatican Council in 1870 to be an article of faith).
"Therefore, to resume, I establish: (1) That Jesus has given to His apostles the same power that He gave to St. Peter. (2) That the apostles never recognized in St. Peter the vicar of Jesus Christ and the infallible doctor of the church. (3) That St. Peter never thought of being pope, and never acted as if he were pope...I conclude victoriously, with history, with reason, with logic, with good sense, and with a Christian conscience, that Jesus Christ did not confer any supremacy on St. Peter and that the bishops of Rome did not become sovereigns of the church, but only confiscating one by one all the rights of the episcopate." (This, along with many arguments against papal infallibility, was said by Bishop Joseph Strossmayer in his speech before the Vatican Council in 1870).
"It has now become quite clear that the conception of continuity, authority, infallibility of the Church and the Church's teaching, on which there has not been sufficient reflection, has led the Catholic Church into a dangerous tight corner." (This, alone with other doubts regarding papal infallibility, was said by Hans Kung, a prominent Catholic theologian, in his book, "Infallibility, An Inquiry," 1971).