The Benedictine Editor argues from Cat. i. 5, “that thou mayest by faith seal up the things that are spoken;” and xxiii. 18:  “sealing up the Prayer by the Amen,” that Cyril means by “this seal” the firm belief of Christian doctrine.  Compare John iii. 33.  But Milles understands by the “seal” the Creed itself, which agrees better with the following context.