St. John Chrysostom (ca. 386), Bishop of Antioch and Constantinople, revered as a "Holy Hierarch of the Orthodox Church," had some nine hundred sermons preserved by church authorities, among which are probably the most inflammatory denigrating attacks ever made on Jews.  In Against the Jews, Oration 1, he proclaims: "Do you see that the demons dwell in their [Jewish] soulsΌthey insult the Master of the prophets himselfΌthey are the common corruption and disease of the whole worldΌwhat manner of transgression haven't they eclipsed by their bloodthirstiness?  They sacrificed their own sons and daughters to demonsΌthey became more savage than any wild beasts.  What else do you want me to tell you — their acts of plunder, greed, their betrayal of the poor, their theft and cheating?" (Mayer and Allen, pp. 161–162). 

Parkes (p. 163): "In [Chrysostom's] discourses there is no sneer too mean, no gibe too bitter for him to fling at the Jewish people.  No text is too remote to be able to be twisted to their confusion, no argument is too casuistical, no blasphemy too startling for him to employ."  St. Jerome (ca. 400), Latin translator of Bibles, enthroned as "Doctor of the Church," declares: "The ceremonies of the Jews are pernicious and deadly, and whoever observes them whether Jew or Gentile, has fallen into the pit of the devil. For Christ is the end of the Torah" (Epistle 75, Michael, p. 21).  "For up to the present day they persecute our Lord Jesus Christ in the Synagogues of Satan" (Epistle 84, NPNF Series 2, vol. 6, p. 176)

As Parkes and others have shown, one reason for these anti-Jewish attacks was Christian attendance at Jewish festivals and rituals.  St. John Chrysostom (Against the Jews Oration 1): "Another very serious disease bids me to speak in order to cure it … What's this disease?  The festivals of the wretched and unsociable Jews are about to approach, thick and fast: the Trumpets, the Tabernacles, the Fasts.  Of the many in our ranks who go to watch the festivals, who say they think as we do, some will both join in the festivities and take part in the Fasts.  This bad habit I want to drive out of the Church right now" (Mayer and Allen, p. 150).

Christian Fathers, beginning with St. Paul (2 Corinthians 3.5–6, see Note #7), long argued that Jewish practices and traditions offer no religious benefit, since Christianity replaced Judaism in God's favor.  A similar attitude of supplanting Judaism is expressed by writers of the Didascalia Apostolorum (ca. 300 C.E.), condemning female Jewish converts to Christianity who continue practicing Jewish water immersion (the "mikvah") to regain religious purity after menstruation. 

Despite such guarded reactions, there is no substantial evidence that Christian participation in Jewish rituals ("heteropraxis") changed Gentile Christianity, making it more Jewish.  Attending a ritual of a different religion does not change the religion being attended nor change the rituals of the attendee's religion.  Even when some renegades/apostates completely circumvent them, barriers between religions remain.  What appears at stake were fears that claims for having replaced Judaism fell on some deaf Christian ears, stimulating Christian leaders, beginning with St. Paul, to respond to any intrusion of Jewish customs vituperatively and illogically. 

Such unaccommodating defensive attitudes helped make the border between Gentile Christianity and Judaism fixed and immutable.  The Sabbath was changed to Sunday, fast days from Mondays and Thursdays to Wednesdays and Fridays, Passover to Easter, and so forth.  In sum, it was the anti-Jewish precepts, doctrines, and contentions, from the time of St. Paul onwards, that held sway through the centuries, reaching an early political apogee when Emperor Constantine legitimatized Christianity (313 C.E.) and Emperor Theodosius (379–395 C.E.) established it as the only sanctioned state religion. 

It is significant that the controversy between Christian Churches about whether the Easter date should or should not accord with Jewish Passover was settled by Emperor Constantine.  His pertinent letter displays the virulent antisemitism induced by his Christian mentors.  "In the first place it was decreed unworthy to observe that most sacred festival in accordance with the practice of the Jews; having sullied their own hands with a heinous crime, such bloodstained men are as one might expect mentally blind. … Let there be nothing in common between you and the detestable mob of the Jews! …Let us with one accord take up this course, right honorable brothers, and so tear ourselves away from that disgusting complicity. … What could those people calculate correctly, when after that murder of the Lord, after that parricide, they have taken leave of their senses, and are moved, not by any rational principle, but by uncontrolled impulse" (Cameron and Hall, p. 128).

Note that even after gaining state power, Christian animus to heteropraxy continued, as we have seen in St. John Chrysostom's and St. Jerome's fulminations.  Stringent governmental laws against Jews and their synagogues (Note #23) then augmented Gentile Christianity's long-sustained non-Jewish/anti-Jewish identity.