Were Christianoi in Antioch Still Observing Yahudi Fasts?

Witness: John Chrysostom · late 4th c.

What the ancient record says

Yes, he repeatedly attacks members of his congregation for participating in Yahudi fasts (his own sermons complain about it). Homily 1 warns that Yahudi festivals and fasts are approaching, says some of his own ranks intend to participate, and states his goal explicitly: drive the practice out of his assembly.

The sermon functions as accidental documentation, condemning the practice proves it existed; the prohibition becomes evidence of the behavior. Does not give a reliable statistical percentage, we can say the practice existed and he considered it serious enough to preach repeatedly against; we cannot say "most Christians in Antioch observed Yahudi fasts" without more evidence.