This evidence is quite sufficient to prove that what St. Paul charged Timothy to do at Ephesus was done not only there but at all the chief centres of the Christian Church: viz., that everywhere great care was taken to provide continuity of authoritative teaching respecting the articles of the faith. It indicates also that as a rule the bishop in each place was regarded as the custodian of the deposit, who was to be chiefly responsible for its preservation. But the precise method or methods (for there was probably different machinery in different places) by which this was accomplished, cannot now be ascertained. It is not until near the end of the second century that we begin to get anything like precise information as to the way in which Christian instruction was given, whether to believers or heathen, in one or two of the principal centres of Christendom; _e.g._, Alexandria, Cæsarea, and Jerusalem. St Paul himself had ruled that a bishop must be "apt to teach" (1 Tim. iii. 2; comp. Tit. i. 9); and although we have no reason to suppose that as a rule the bishop was the only or even the chief instructor, yet he probably selected the teachers, as Timothy is directed to do here. In the great Catechetical School of Alexandria the appointment of what we should now call the Rector or senior professor was in the hands of the bishop. And, as we might expect, bishops selected clergy for this most important office. It forms one of the many contrasts between primitive Christianity and heathenism, that Christians did, and pagans did not, regard it as one of the functions of the priesthood to give instruction in the traditional faith. The heathen clergy, if consulted, would give information respecting the due performance of rites and ceremonies, and the import of omens and dreams; but of their giving systematic teaching as to what was to be believed respecting the gods, there is no trace. It is more than probable that a great deal of the instruction both to candidates for baptism and candidates for the ministry was from very early times reduced to something like a formula; even before the dangers of corruption arising from Gnosticism rendered this necessary, we may believe that it took place. We know that the Gospel history was in the first instance taught orally; and the oral instruction very soon fell into something that approached to a stereotyped form. This would probably be the case with regard to statements of the essentials of the Christian faith. In Ignatius (_Philad._, viii.), Justin Martyr (_Apol._, I. 61, 66), and in Irenæus (_Hær._, I. x. 1) we can trace what may well have been formulas in common use. But it is not until the middle of the fourth century that we get a complete example of the systematic instruction given by a Christian teacher, in the Catechetical Lectures of St. Cyril, Bishop of Jerusalem, delivered, however, before his episcopate.