Page "Ambrosian Rite" Paragraph 2

There is no direct evidence that the rite was in any way the composition of St. Ambrose, but his name has been associated with it since the eighth century at least.

It is probable that in his day it took a form which included the principal characteristics distinguishing it from other rites but has since been subject to various revisions from time to time.

St. Ambrose succeeded the Arian bishop Auxentius of Milan, during whose long episcopate ( 355 to 374 ) it would seem probable but unverified that Arian modifications may have been introduced into a rite the period of whose original composition is unknown.

It would be sufficient cause to attach St. Ambrose's name to the rite if St. Ambrose expunged these hypothetical unorthodoxies and issued corrected service books.