In Biblical times, Jewish national identity was both religious and cultural. They believed they were set apart by God, entrusted with His law, and tasked with worshiping Him. It was a total, all-consuming, non-negotiable reality, directing every aspect of their lives. The Jewish people would rather die than abandon their faith, and many did.
Someone from a Jewish background leaving the tenants of Judaism, and being converted to a follower of Jesus, at that time would be considered miraculous – not to mention the convert would be accused of blasphemy and potentially subject to death by stoning.