I'm a big fan of Schubert's liturgical and sacred works.
It is well known that Schubert had some doctrinal issues with the some parts of the Mass. Like Beethoven, he was brought up in the RC faith but neither of them was a regular Church service attendee. In Schubert's case his attachment to religion was somewhat quirky, but not overall questioning.
Schubert churned out loads of liturgical and sacred music, not just 6 complete masses but various stand-alone parts of the Mass like the Kyrie, Sanctus, Benedictus, Offertorium. He also wrote several pieces that were formed the accompaniment of other services like the Benediction and the Rosary, e.g. the Tantum Ergo, Stabat Mater, Magnicat, Salve Regina. He wrote very beautiful settings for psalms 23 and 92, and an Oratorio,"Lazarus". He even wrote a Requiem Mass that not many know about, D 453 Requiem in C minor, which was unfinished, and ends abruptly.
The first four of Schubert's Masses are quite short. The last two, Masses 5 and 6 are much longer. Mass No 5 in A-flat was composed between 1819 and 1822, and then revised several years later. The Mass No 6 in E-flat major, D 950, was written in 1828, the year of his death and was part of Schubert's last great musical efforts, the size and high quality of which must go down as the most monumental last year achievements in the whole of classical music history.
I haven't studied the exact text in all of Schubert's 6 Masses but I understand that he deliberately left out parts of the Creed that caused him concern. He regularly left out “Et unam, sanctam, cathólicam et apostólicam Ecclésiam", presumably because he didn't agree with it. In what many consider to be his best and most famous Mass, Mass 6 in E flat, he also left out several other parts of the Creed: “Patrem omnipotentem”; “Génitum, non factum, consubstantiálem Patri"; and “Et expecto resurrectionem mortuorum". I believe that the Gloria in Mass 6 also is truncated by: “... súscipe deprecatiónem nostram. Qui sedes ad déxteram Patris, miserére nobis".
These various excisions got Schubert into a bit of bother with the Church authorities of the day, and subsequently, but I don't think they necessarily caused Schubert's Mass settings to be set aside completely. Rather I think that some of them were performed.
Mass 6 is one of my favourite Schubert works. I think it is the most beautiful Mass setting of the whole lot, including any by Beethoven, Mozart, Haydn, or by anyone elsee. To me, despite its various excisions, it sounds spot-on in terms of getting just about everything right in terms of beauty, balance and piety. Performance is everything here. I have several copies of the work and my favorite version is by Sawallische with the Bavarian Radio Symphony Orchestra, and an all-star line up of vocalists.