In the collection of Rav Soloveitchik’s responsa Community, Covenant and Commitment, there is a series of letters railing against mixed seating in the synagogue. Amongst the points raised there is a claim made in letter 18, pg 134. I quote:
When primitive Christianity arose as a sect in the Holy Land, and began to slowly introduce reforms, one of the innovations which the sect established at once in the externals of synagogue practice was to have men and women sit together. In many instances mixed seating was the unmistakable sign by which a Jew could recognize that he had found not a place of sanctity for Jews to pray, but rather a prayer-house for a deviant sect; for in those times the Christians had not yet formally differentiated themselves from traditional Jewry. As a sect they endeavored to hide their identity, and only through certain definite signs could they be recognized.
Where is the source for this?