I never understood that concept. The idea that Judaism is transmitted from lineage is itself only a belief of Judaism. If you don't believe in Judaism there is no reason to believe in this particular fiction. This specific feature is obviously a hack on the reproductivity of the meme. There is no Judaism gene, it's not transmittable, it's not an ethnicity.

No problem! It certainly doesn't make sense if one approaches it from a Western Christian mindset. I am not trying to imply that is how you are approaching it, rather I mention this because Judaism often gets compared to Christianity, but I think they are quite different.

It is better to think of Judaism as a tribe. Membership to this tribe is mainly given under two conditions: through matrilineal descent or through religious conversion. Some subgroups within the tribe have different membership conditions like recognizing patrilineal descent, others may not recognize conversions done by other subgroups.