What has Christianity truly contributed to humanity

[Bucky Ball]

Not sure I agree. He wrote a whole bunch of other (secular) stuff also, (Brandenburg Concertos example), [..]

Yeah, I mentioned that already, thanks; we're in agreement on that. [See emphasis above]

... and I have no information if he was "devout" or not, so I don't know what the inspiration was.

I didn't say his only inspiration was Christianity.  But he attended Lutheran church from boyhood. He played for churches throughout his adult life.

He produced several church cantatas at this time; all of these works are cast in a conservative mold, based on biblical and chorale texts and displaying no influence of the “modern” Italian operatic forms that were to appear in Bach’s later cantatas. The famous organ Toccata and Fugue in D Minor (BWV 565), written in the rhapsodic northern style, and the Prelude and Fugue in D Major (BWV 532) may also have been composed during the Mühlhausen period, as well as the organ Passacaglia in C Minor (BWV 582), an early example of Bach’s instinct for large-scale organization. Cantata No. 71, Gott ist mein König (God Is My King), of February 4, 1708, was printed at the expense of the city council and was the first of Bach’s compositions to be published.

[...]

As director of church music for the city of Leipzig, Bach had to supply performers for four churches. At the Peterskirche the choir merely led the hymns. At the Neue Kirche, Nikolaikirche, and Thomaskirche, part singing was required; but Bach himself conducted, and his own church music was performed, only at the last two. His first official performance was on May 30, 1723, the first Sunday after Trinity Sunday, with Cantata No. 75, Die Elenden sollen essen. New works produced during this year include many cantatas and the Magnificat in its first version. The first half of 1724 saw the production of the St. John Passion, which was subsequently revised. The total number of cantatas produced during this ecclesiastical year was about 62, of which about 39 were new works.

On June 11, 1724, the first Sunday after Trinity, Bach began a fresh annual cycle of cantatas, and within the year he wrote 52 of the so-called chorale cantatas, formerly supposed to have been composed over the nine-year period 1735–44. The “Sanctus” of the Mass in B Minor was produced at Christmas.

https://www.britannica.com/biography/Joh...tian-Bach/

I think it's fair to say that a piece titled "Jesu, Joy of Man's Desire" has religious inspiration at its root.

Do you have any objective reason for doubting his faith, or the music it inspired?  Do you have any sources one might read about this?

It's not "fair". You obviously don't know any church musicians, and me, I know too many.  
He needed a piece, found a text, (or was given it), and wrote it out. Your assumption is totally unwarranted. 
He needed something to fill out a service, just like countless others.
I might not doubt his faith, but I know of no reason to assume his work-product or even occupation was in any way the result of any "faith".
Why is a Lutheran writing Catholic masses ?