Matrilineal religious Judaism was instituted in or around 200 ad.
That said, your statement presumes that no men were Jewish and we know that's obviously false. In fact, familial names, at least in historic times often representing tribes, were naturally patrilineal and, of course, there were Jewish mean. Hence Y-DNA tracing is an extremely effective of tracing that heritage.
People have been named Levy and Cohen for 1000 years, for example. You are saying that tracing their familial Y-DNA is not effective for tracing their Jewish cultural heritage? Hmmm.. I think the founder of familytreedna.com, a Jewish guy, would disagree. His colleague and board member was the guy who won the nobel prize for exactly this testing.
Not to mention if you read actual geneological records of families whose histories were traced via Y-DNA, they line right up. I'm looking at one now that traces an Ashkenaz family back to Portugal in the 1400s. The family history and the Y-DNA results are virtually synonymous.
While the 1400s are not the 400s, it does speak to the fact that Y-DNA tracing is extraordinarily accurate particularly in Jewish families who, culturally, had very little intermixing with other cultures until the current era.
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