This was amazing because, from a worldly perspective, Paul’s relationship with the Thessalonians was a very unlikely one. Paul was a former Pharisee, known for his strict adherence to the Law of Moses as well as the oral law established by various rabbis. A big part of being a Jew, especially a Pharisee, was maintaining religious and ritual purity. This involved what he could eat or not eat, what he could do or not do on the Sabbath, and even what he could wear and not wear. Even now, when you go to grocery stores, you have food items labeled as “Kosher.” That means it satisfies the dietary restrictions that are required by the Law of Moses.
These ceremonial regulations obviously affected a Jew’s relationship with non-Jews, who did not follow the same regulations and rituals: the Gentiles were considered unclean. So, the Jews were not allowed to marry non-Jews. In principle, this was more about Israel’s religious purity than ethnic purity. Like Rahab and Ruth, those who embraced Israel’s religion were welcomed into the covenant community.
But in practice, it often degenerated into ethnic discrimination. In some cases, the Gentiles were referred to as “dogs” because of their religious idolatry and ritual impurity. Think about what the Samaritan woman said when Jesus asked for water: "’How is it that you, a Jew, ask for a drink from me, a woman of Samaria?’ (For Jews have no dealings with Samaritans)” (John 4:9). We also hear of the incident in Antioch involving Peter: “before certain men came from James, [Peter] was eating with the Gentiles; but when they came he drew back and separated himself, fearing the circumcision party” (Gal. 2:12). This implies that Jews did not eat with Gentiles--so wide was the divide between Jews and Gentiles; so high was the barrier between the two groups.