Understanding Peirce requires dealing with Peirce's religious concerns, which are increasingly recognized as being as philosophically relevant as his scientific concerns. In recent times, even Peirce's regular religious practice in his Milford years has been documented (L 244), including, at least occasionally, week-day Eucharist services, which were "the hallmark of Tractarian or Anglo-Catholic parishes". -/- I have argued elsewhere that for Peirce, scientific activity is a genuine religious enterprise, perhaps even the religious activity par excellence, and that to divorce religion (...) from science is antithetical to both the scientific spirit and the genuine Peirce. In this vein, I have also held that Peirce's framework for the relations between science and religion, reason and faith, seems congenial to the Roman Catholic tradition. Perhaps the strongest conflict between Peirce's view on science and Roman Catholic faith may be epitomized in the dogma of papal infallibility, declared by the Vatican Council I and Pius IX on July of 1870, only eleven weeks before Peirce's first visit to Rome. Since the first moment, papal infallibility has been a permanent object of mockery and derision in the cultivated circles of Anglo-American intellectuals. As the late Rorty wrote, Pius's decision "was making Catholicism look ridiculous". -/- In this broad framework, the aim of my paper is to provide some context for Peirce's letter about papal infallibility, as the doctrine was presented by his former colleague George M. Searle in his 1895 book "Plain Facts for Fair Minds", which Peirce came across almost by chance. According to Peirce, there is a deep contrast between the genuine scientific attitude and the putative metaphysical notion of 'absolute truth' that was behind Searle's defense of infallibility. "I would with all my heart join the ancient church of Rome if I could. But your book," —Peirce writes to Searle (L 397) — "is an awful warning against doing so." -/- In order to explain Peirce's position, the paper is arranged into four sections: 1) a brief presentation of George M. Searle and his book Plain Facts for Fair Minds; 2) a description of Peirce's letter to Searle; 3) Peirce's fallibilism and infallibility; and 4) an attempt to guess how Searle might have responded to Peirce. I will try to collect some of Peirce's texts and to quote them extensively, since it is possible to learn a lot from the exploration of this debate. (shrink)