In the time of Christ, there was a high degree of prejudice against non-Jews who wanted to worship God. Gentiles were not allowed to participate in worship at the temple in the same way as the Jews and were not seen by the Jews as spiritual equals. Modern Judaism does not carry this prejudice. They maintain that "converts" have the same status as those who are born Jewish, commonly citing King David as an example of a “convert” who was righteous and did many great things in the scriptures.
But wait! David was the son of Jesse, the son of Obed, the son of Boaz, the son of Salmon, the son of Nahshon, the son of Amminadab, the son of Ram, the son of Hezron, the son of Perez, the son of Judah!!! (I knew all of those genealogies would come in handy someday!) Since these men were all Israelites from the tribe of Judah, why do they say that David was a “convert?”