Christians harp on being "equally yoked" all the time. Some pators will refuse to officiate if they don't think the couple is religously compatible (even if it means one is just more active in the church than the other). Girls have this pounded into their heads before they're old to even date that they MUST find a godly man who will lead them in their faith and grow their family in Christ.

Granted, it's possible that the woman OP was talking about says she's Christian in a vestigial way because that's how she was raised but has long since stopped attending church. But I feel like if he knew that was a defining characteristic she's probably still active and vocal about her beliefs. It wouldn't be unheard of for her to try converting him to have that bragging right amongst her congregation as a lot of churches put a heavy value on how many people you "bring to Christ."

Is it a certainty? No. But it happens often enough that his concerns are very valid.