The Apostate’s Dilemma: How Petrus Alfonsi’s Conversion and Dialogue Framed Jewish-Christian Debate

What did the Jewish people of his time mean by bad faith and what were the arguments about? Like many converts of his time, Alfonsi was accused of bad faith by the Jewish community and to counter this, as well as to show his zeal for his new faith he wrote a work attacking Judaism and defending Christianity. It became one of the most widely read and used anti-Jewish polemical texts of the Middle Ages,[13] as Tolan shows. Alfonsi wrote the Dialogues in 1110; he presents them as a disputation between his former Jewish self (Moses) and his current Christian self (Peter). He divides the work into twelve “Dialogues” or chapters: and the first four attack Judaism, the fifth attacks Islam, and the last seven defend Christianity.

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In this context “bad faith” isn’t a technical medieval Jewish term; it’s a modern historian’s way of summing up how Jews viewed someone like Moses Sephardi / Petrus Alfonsi after his conversion.

What Jews of his time meant by “bad faith”

For Jews in the 11th–12th centuries, a Jew who converted to Christianity was not just someone who “changed opinions.” He was an apostate (meshummad / mumar) — someone who had:

Broken the covenant with God and Israel,
Betrayed his own community, often in favor of the very powers that oppressed Jews,
And often used insider knowledge to harm Judaism in front of Christians.Jewish Encyclopedia+2UltimateLexicon.com+2

So when modern summaries say Alfonsi “was accused of bad faith by the Jewish community,” they’re getting at several layers:

Insincere motives
- He converted under royal patronage (Alfonso I of Aragon) and later served as court physician to Henry I of England.Wikipedia
- Jews could easily read this as: he didn’t embrace Christianity out of pure conviction; he did it for status, safety, and patronage.
Breach of loyalty / treachery
- Apostasy was seen as desertion from both religious and national solidarity with Israel. Medieval Jewish texts often describe apostates as bringing “ruin upon body and soul” and as people who sell out for worldly advantage.Jewish Encyclopedia+1
Speaking against Judaism as an insider
- Alfonsi didn’t just quietly leave; he then wrote an influential Latin treatise against Judaism, explicitly using his background as a Jew to give weight to his attacks.Wikipedia+1
- That’s precisely what Jews found “in bad faith”: he leveraged their trust and shared learning to arm their opponents.

So “bad faith,” from their perspective, means a betrayal cloaked in claimed religious zeal: a Jew who turns Christian and then aggressively attacks Judaism to prove his new loyalty.

What were the arguments actually about?

Alfonsi’s Dialogi contra Iudaeos (“Dialogues Against the Jews”, c. 1110) is staged as a debate between:

Moses – his former Jewish self
Peter – his new Christian selfWikipedia+1

He divides it into twelve dialogues:

Dialogues 1–4: Attacks on Judaism
- Authority of the Talmud: Alfonsi argues Jews no longer follow the true “Old Law” of Moses, but a “new and heretical law” — the Talmud — which he calls a “fabric of lies” written to hide that Jesus is the Son of God.Wikipedia
- Charge against the rabbis: He claims rabbinic leaders know Jesus is the Messiah and Son of God but have deliberately misled the people, out of envy and a desire to retain power.Wikipedia+1
- Exile and punishment: He says Jewish exile is divine punishment specifically because “the Jews” knowingly killed Christ — one of the first Latin polemics to frame Jews as consciously guilty of deicide.Wikipedia
From a Jewish perspective, these are gross misrepresentations of rabbinic teaching and history, presented as if an “insider” were revealing secrets. That’s a classic reason to see him as acting in bad faith.
Dialogue 5: Attack on Islam
- Moses (the Jewish self) asks why Peter didn’t choose Islam, and praises Islam’s virtues; Peter then argues Christianity is superior to Islam as well. Alfonsi uses this to link Judaism and Islam as allied errors against Christianity.jewish philosophy place+1
Dialogues 6–12: Defense of Christianity
- Christological readings of the Hebrew Bible: Peter uses the Hebrew Scriptures to prove the Incarnation, the Trinity, and Jesus’ Messiahship.
- Natural law and reason: He frames Christian doctrine as rational and aligned with natural law, while portraying Jewish interpretation as willful blindness.Scholars’ Bank+1

So, the arguments themselves are about:

Who rightly interprets the Hebrew Bible,
Whether rabbinic Torah (Talmud) is faithful to Moses or a corruption,
Whether Jews are guilty of knowingly killing the Son of God,
And whether Christianity (vs. Judaism and Islam) is the true and rational faith.

From the Jewish side, calling him “in bad faith” is shorthand for:

You abandoned the covenant, gained power and favor from our enemies, and now use your insider knowledge to slander our faith and leaders for their benefit.

If you’d like, I can help unpack how this compares or connects to later accusations of “bad faith” in confessional polemics (e.g., Reformation-era debates or Luther’s rhetoric).