Eusebius praises the pastoral fidelity of the primitive pastors, in their unwearied labours to protect their flocks from the heresies with which Satan contrived to endanger the souls of believers. By exhortations and admonitions, and then again by oral discussions and refutations, contending with the heretics themselves, they were prompt to ward off the devouring beasts from the fold of Christ. Such is the praise due to Theophilus, in his opinion; and he cites especially his lost work against Marcion as "of no mean character."

He was one of the earliest commentators upon the Gospels, if not the first; and he seems to have been the earliest Christian historian of the Church of the Old Testament. His only remaining work, here presented, seems to have originated in an "oral discussion," such as Eusebius instances. But nobody seems to accord him due praise as the founder of the science of Biblical Chronology among Christians, save that his great successor in modern times, Abp. Usher, has not forgotten to pay him this tribute in the Prolegomena of his Annals.