He later earned a doctorate in philosophy in the philosophy of religion from Fordham University.[11][12][13] While studying at Old Dominion University, he was challenged to convert to Islam by his roommate, Nabeel Qureshi (an American Muslim of Pakistani descent), and went about investigating the life of Muhammad using the earliest sources, including Ibn Ishaq's Life of Muhammad (the earliest extant biography of Muhammad); the hadith collections of Sahih al-Bukhari and Sahih Muslim (considered by Sunni Muslims to be the two most reliable or sahih collections of Muhammad's statements, actions, and example); and the History of the Prophets and Kings by Al-Tabari (one of Islam's greatest historians).[11] Concluding that the Quran and Muhammad's example did not simply describe violence in the past (as in the Christian Bible per his assertion), but rather commanded ongoing violence, he then became a Christian apologist.[11] His roommate Nabeel eventually went on to become a Christian apologist as well.[5]