Why is it significant that every writer of Scripture was Jewish?
It is significant because God committed the oracles of God to the Jewish people; every word of Scripture came through Israel. Paul explains in Romans 3 that the Jew's advantage was chiefly that they were entrusted with God's Word, making the Bible unique from all other religious books.
Les teaches that everything had to come through this one little nation, Israel, and every word of this Book came out of the nation of Israel. He notes that all the writers of Scripture, including Luke, were Jews, and Paul makes this plain in Romans chapter 3: “What advantage then hath the Jew?” The answer is “Much in every way, but especially or chiefly, that unto them were committed the oracles or the word of God.” This is what makes this Book totally unique from any other religious book on this planet. It also fits the Abrahamic Covenant, where God promised Abraham that in his seed all the nations of the earth would be blessed.