The fact, then, of such doctrinal development being conceded, the question arises: How is it to be accounted for? What are its causes? Why, in other words, should there have been any such development at all? Why, rather, should not the exhibition of Christian doctrine and devotion have appeared uniformly one and the same at all times from the beginning?

Many reasons might be adduced to account, in part at least, for a certain reserve during the first centuries of Christianity, adn also for a subsequent corresponding development in religious teaching. Of these we shall notice what appears to us to be the principal.

1. There was in primitive times what was called the discipline of the secret (disciplina arcani), whereby, especially for the sake of reverence, a reserve was practiced in the public teaching of the faith, and certain portions of the Church's system were held back, lest by their publicity holy things should be profaned by the heathen, and catechumens be prematurely initiated in the more sacred Christian mysteries. This reserve, as time went on, became neither useful nor practicable; and hence the Christian religion showed itself later on in a more developed form.*

2. It was simply impossible to explain fully and set forth in detail the whole circle of revealed truths all at once from the beginning. Considerable time was necessary for such exhibition. The Apostles and those who succeeded them, from an unwillingness to overburden new converts, would in their preaching and instruction propose and insist upon those truths first of all which were most essential, as belonging to the substance of the Christian faith, and most important for practice; whilst they would touch lightly on accessory truths as being less necessary to be explicitly known, or would hold them in reserve for some more favourable occasion. They would, on the other hand, where special circumstances required it, dwell on certain secondary points of doctrine with greater stress and more at length than their intrinsic importance might seem to demand.

* "There was a time when Christians only, and not even catechumens, knew what is present on our altars. By degrees, heretics also knew what the Church believed thereon. Now any rationalist philosopher can tell you what is believed by a Catholic when he talks of the Real Presence. Here an order is observed: God taught His Church from the first, what at the last all the world has come to know. God let the awful secret escape, so to speak, by degrees. First, men dreamt a child was slaughtered in the rites of Christians; by degrees they knew something more was there, and of a more mysterious kind; now all men know that we believe that the Flesh and Blood and Soul of God the Son is present on our altars. In a similar way, the body of her from whom he took that flesh was first thought to have been defiled by adultery; then she was thought to have fallen as low as to have other children after God; then she was proclaimed Mother of God; then other titles of honour came before the world; and now heretics and heathens can learn, that we claim for the soul and body of Mary absolute immunity from sin." -- J.B. Morris, Jesus, the Son of Mary, vol ii. p. 365, 1851.