His opposition to Christianity was not however based wholly on a prejudice of feeling. He was a man cultivated in all the learning of his age, and of a more generous temper than Celsus, and seems to have exercised much critical sagacity in the investigation of the claims of Christianity. About the year 270, while in retirement in Sicily, he wrote a book against the Christians.(200) This work having been destroyed, we are left to gather its contents and the opinions of its authors from a few criticisms in Eusebius and Jerome. The entire work consisted of fifteen books; and concerning only five of these is information afforded by them. Their remarks lead us to conjecture that it was an assault on Christianity in many relations. The books however of which we know the purpose, seem to have been critical rather than philosophical, directed against the grounds of the religion rather than its character; being in fact an assault on the Bible. The existence of such a line of argument, of which a trace was already observable in Celsus, is explained by the circumstance that the faith of Christendom was already fixed on the authority of the sacred books. The church had always acknowledged the authority of the Jewish scriptures; and by the middle or close of the second century at the latest, it had come to acknowledge explicitly the co-ordinate authority of a body of Christian literature, historic, and epistolary.(201) Hence, when once the idea of a rule of faith had grown common, the investigation of the contents of the scriptures became necessary on the part of heathen opponents. The growingly critical character of Porphyry’s statements, though partly attributable to the literary culture of his mind, is a slight undesigned evidence corroborative of the authoritative nature already attributed to the scriptures in doctrine and truthfulness. Porphyry seems accordingly to have directed his critical powers to show such traces of mistakes and incorrectness as might invalidate the idea of a supernatural origin for the Jewish and Christian scriptures, and shake confidence in their truth as an authority.