Laudator Temporis Acti

Brian Newbould, Schubert: The Music and the Man (Berkeley: University of California Press, 1999), pp. 35-36 (footnote omitted):

At the same time, whatever his convictions amounted to, he was evidently unable to align them with the tenets of the institutionalized church. This much is an almost inevitable conclusion to be drawn from his free editing of the mass text, in various of his settings. A regular feature of this editorial licence is the omission of the words 'Et in unam sanctam catholicam et apostolicam Ecclesiam'. Had he not consistently omitted these words, from all his six settings, one might put the matter down to carelessness or suppose that he called the text to mind (from his regular singing of it) without checking it and his memory failed him. Equally, it could be imagined that by 'thinning out' the text of the Credo Schubert was hoping to reduce the disparity between the syllabic treatment that was desirable and traditional in a movement with a long text and the necessarily melismatic treatment of, for example, the Kyrie. There does remain a possibility that he found these words musically unpromising, not because of their connotation but because of their implied rhythm or the length of the verbal phrase as a whole; and that aesthetic objection to a particular section of text would then explain why his rejection of it was invariable. There are, on the other hand, phrases elsewhere in the Credo (and the Gloria, for that matter) that a composer might find somewhat intractable musically and feel tempted to omit; but Schubert omitted no other passage consistently. The line concerned is, of course, an article of the creed; and in this connection it is relevant to refer to an invitation Schubert received from a friend, Ferdinand Walcher, in 1827, which began with the words 'Credo in unum Deum!' (set to the notes of the Gregorian intonation) followed by 'You do not, I well know; but you will believe this — that Tietze is going to sing your Nachthelle [Night's Brightness] at the Little Society [i.e. Philharmonic Society] this evening.' This seems a clear enough suggestion that Walcher knew that Schubert did not swallow the creed hook, line and sinker, and although it may be wondered if documents from the last three years of Schubert's life are sure reflections of his outlook in 1814, the fact that his first mass makes the textual omission in question, as does the last one (the E flat of 1828) and all those between, rather points to an unchanging viewpoint in this respect on Schubert's part.

The omitted words mean "and in one holy, Catholic, and apostolic Church."