It is probably that to many, but certainly not to all.

So is Handel`s, Mozart`s Beethoven`s. ETC

Yes, this is an important point that answers why a Luteran composer composed a Catholic Mass.

I

Elzenberg Hmm, the short masses were written for the court, and for one of those he received title of court composer. B minor mass is too long to be used in a service…

The problem of this argument is that Bach in fact never wanted to work in Dresden, some years late he was called for the office in Dresden and didnt accept, stayed in Leipzig. When he was completing the last parts of "Second" Mass, the catholic one, he hadnt health anymore to change his life with family to another city. He was showing signs of vision lost when was completing the last parts of the mass.

Bach also wrote cantata 198 for the funeral of the Queen of Poland.

It is more complicated than that. The first part of the mass, Kyrie plus Gloria was composed for a job opportunity, although Bach used it to put pressure on Leipzig more than to get the job himself. It is not this Mass that he speaks of. Years later he decided to complete the mass with the final parts, III. Credo ("Symbolum Nicenum") IV. Sanctus, Osanna, Benedictus, V. Agnus Dei and Dona Nobis Pacem and it´s about that complete Mass he is talking about. Bach really didn´t have any obligation to complete these final movements and didn´t get any money for that. In fact the entire B minor mass was performed the first time in full form much later, after Bach's death, so there was no way he could receive any payment for it. He used some of his ancient compositions for the Mass, indeed, but not for all parts. And he had to reworked all movements, it was not a copy and paste.

I am aware of one other part of the mass as being written for the mass.

@@shnimmuc I think you're simplifying too much, look. Credo in unum Deum (the movement 1 from Credo) - Five-part chorus (Soprano I & II, Alto, Tenor, Bass) is not gotten from any ancient Bach composition. "Et incarnatus est" was specially composed for the Mass, There is no evidence that "Et resurrexit" was gotten from any ancient work. If you have the source that Bach used of old compositions for these parts, please say me, in my sources I found that, and I am not perfect. (But I am sure that "Et in incarnatus est" was specially composed for the Mass, not gotten from any old work. There is something interesting that few people know, the Kyrie 1 (first movement of the mass) was based in a Kyrie of another composer, who even was not a good composer, Johann Hugo von Wilderer. It ´s a Bach composition but it is very surprising that Bach got the ideas from second-class composer for a so important composition (well, Mozart got the first 2 movements of his Requiem from Handel works.) Bach obviosly improved the original ideas and composition of Wilderer. You can hear in ( it is just the Wilderer Kyrie, not the entire mass) kzfaq.info/get/bejne/sL1pnLR_yr_Qp2w.html and here in other recording kzfaq.info/get/bejne/prCTlZWS29zanmw.html . It is not just the introduction, the 4 or 5 first note of the vocal part too, the subject of the fugue which follows, the repetition of them and the rhythm. After the Wilderer (or Bach) change the vocal part. " "Johann Sebastian Bach copied and performed Wilderer's Missa brevis in G minor in around 1730, and for a while the work was believed to have been composed by Bach himself. Christoph Wolff noted similarities between a theme from Wilderer's Missa and the theme of the Kyrie I from Bach's Mass in B minor as well as some similarities in the structure of both works."

@@codonauta I will do some research and what you mentioned. However because of Covid, I will not have access to the libraries at the university I teach at, so it will take some time. I hope you were not referring to Mozart`s borrowing from Handel as a first rate composer borrowing from a second rate one. I consider Handel (not Mozart) to be an apex composer, right there with Bach.

@@codonauta This is silly to go on about this subject. The mass was not composed from A-Z and you cannot change it.