12 But in learning the Faith and in professing it, acquire and keep that only, which is now delivered to you by the Church, and which has been built up strongly out of all the Scriptures. For since all cannot read the Scriptures, some being hindered as to the knowledge of them by want of learning, and others by a want of leisure, in order that the soul may not perish from ignorance, we comprise the whole doctrine of the Faith in a few lines. This summary I wish you both to commit to memory when I recite it, and to rehearse it with all diligence among yourselves, not writing it out on paper, but engraving it by the memory upon your heart, taking care while you rehearse it that no Catechumen chance to overhear the things which have been delivered to you.
I wish you also to keep this as a provision through the whole course of your life, and beside this to receive no other, neither if we ourselves should change and contradict our present teaching, nor if an adverse angel, transformed into an angel of light should wish to lead you astray. For though we or an angel from heaven preach to you any other gospel than that you have received, let him be to you anathema. So for the present listen while I simply say the Creed, and commit it to memory; but at the proper season expect the confirmation out of Holy Scripture of each part of the contents.
For the articles of the Faith were not composed as seemed good to men; but the most important points collected out of all the Scripture make up one complete teaching of the Faith. And just as the mustard seed in one small grain contains many branches, so also this Faith has embraced in few words all the knowledge of godliness in the Old and New Testaments. Take heed then, brethren, and hold fast the traditions which you now receive, and write them on the table of your heart.
Source: Catechetical Lectures (New Advent)