In 1733, Bach wrote a letter petitioning Friedrich August II, the Catholic Elector of Saxony, to grant him a courtly title that might be of value to him in getting his due respect from the powers in Leipzig. To warm up the sovereign to his cause, he enclosed two pieces of music as special proof of his dedication to the composition of church music; these pieces were the Kyrie and Gloria of the Mass in B minor, a juggernaut of religious music that Bach didn't complete until the very end of his life. Why a composer who must have normally worked at blinding speed took 15 years with a single mass is not known, since there was no opportunity for its performance. As a mass, it's far too vast for liturgical use, and earnest religious music couldn't have been welcomed in secular, courtly programs. It definitely wasn't entirely performed while Bach lived, but it seems possible he didn't intend it to be performed that way at all. Many movements are highly effective revisions of past works, often cantatas, spanning much of his career, and the others were composed expressly for the mass. These facts, and the wide differences in style the work contains, suggest it was intended as a summation of his whole oeuvre, but that can never be known.
Of course, the mystery of its purpose and origins have fed the fire of enthusiasm that surrounds the mass. For once, the hype is mostly worth believing. Commentators stumble over each other to praise it, treating it like a St. Peter's of music for good reasons; the Mass in B minor positively crackles with energy, and almost everything that is good about Bach is found in it. Hearing even the brooding Kyrie for the first time can be like having of a pair of jumper cables applied to the heart. Unfortunately, the size, scale, and historical importance of the mass, taken together, seems to confuse certain interpreters into performing it with the overblown orchestral forces and exaggerated expression of late Romantic music. It becomes an overstated banality when treated that way; smaller orchestras can bring out of it an amazing, galvanized lyricism and mechanical power. The range and depth of moods is itself incredible enough; listeners almost prefer to hear the movements discretely to be able to properly take them in. From the most ecstatic, trumpeting orchestral jubilation of the start of the Gloria, to the tender, pained longing in the soprano and tenor duet of the Laudamus te, or the unstoppable fugue of the Cum sancto Spiritu, the Mass in B minor is as exhilarating to the listeners as it is exhausting to performers. Some lighter, simpler choral movements, like the Gratias agimus tibi, have a minor function of granting needed rest, but there don't seem to be quite enough of them to make it really functional as a concert piece. For the highly trainable medium of the compact disc, however, it's just right.