11 Let me give you this charge also. Study our teachings and keep them for ever. Think not that they are the ordinary homilies; for though they also are good and trustworthy, yet if we should neglect them today we may study them tomorrow. But if the teaching concerning the laver of regeneration delivered in a consecutive course be neglected today, when shall it be made right? Suppose it is the season for planting trees: if we do not dig, and dig deep, when else can that be planted rightly which has once been planted ill?
Suppose, pray, that the Catechising is a kind of building: if we do not bind the house together by regular bonds in the building, lest some gap be found, and the building become unsound, even our former labour is of no use. But stone must follow stone by course, and corner match with corner, and by our smoothing off inequalities the building must thus rise evenly. In like manner we are bringing to you stones, as it were, of knowledge. You must hear concerning the living God, you must hear of Judgment, must hear of Christ, and of the Resurrection.
And many things there are to be discussed in succession, which though now dropped one by one are afterwards to be presented in harmonious connection. But unless thou fit them together in the one whole, and remember what is first, and what is second, the builder may build, but you will find the building unsound.
Source: Catechetical Lectures (New Advent)