Complex sentence: Ruth convinced the singers by arguing that, since the Apollo was a den of iniquity, that is exactly where a gospel group should sing to bring the sacred message, and that the building itself should not matter if they were there to worship God.
Split into simpler sentences: Ruth convinced the group by arguing that, since the Apollo was a sinful den of iniquity, that was exactly where a gospel group should sing. There they could bring the sacred message to the sinners, and that the building itself should not matter if their performance of gospel was to worship God.