CCC 186 From the beginning, the apostolic Church expressed and handed on her faith in brief formula normative for all.1 But already very early on, the Church also wanted to gather the essential elements of her faith into organic and articulated summaries, intended especially for candidates for Baptism:
This synthesis of faith was not made to accord with human opinions, but rather what was of the greatest importance was gathered from all the Scriptures, to present the one teaching of the faith in its entirety. And just as the mustard seed contains a great number of branches in a tiny grain, so too this summary of faith encompassed in a few words the whole knowledge of the true religion contained in the Old and the New Testaments.2
1 Cf. Rom 10:9; I Cor 15:3-5, etc.
2 St. Cyril of Jerusalem, Catech. illum. 5, 12: PG 33, 521-524.