Although he didn't date it we know from the original autograph that Haydn's Missa brevis Sancti Joannis de Deo or the 'Kleine Orgelmesse' as it's also called, dates from the 1770s. The Sancti Joannis de Deo (Saint John of God) in the title is a reference to the patron saint of the Hospitallers of St John of God, commonly referred to as the St. John of God Brothers an order of monks who specialise in medical services. They believed strongly in music as a palliative which is why music played such a strong part in their religious services. Haydn was well acquainted with the order he'd played the violin in their Viennese church during the 1750s and had composed numerous small pieces for them during his youth. If you ever come across early Haydn pieces that deal with an Advent theme the chances are it was written for the Barmherzigen Brüder. Nor did his connections cease as he grew older as the Eszterházy's were generous and regular benefactors of the order.
Based on the sparsity of the musical forces for which he wrote it I think it most likely that Haydn wrote this Mass for performance in the small church in Eisenstadt. The 'Missa brevis' in the title means that it's for routine services that is for performance on days weren't important feast days or celebrating a patron's nameday. The word 'brevis' means 'short' or 'brief' and there are several techniques a composer can make use of to achieve brevity. One of them is polytextuality a technique in which several clauses of the lenghtier texts in the Mass are sung simultaneously if you listen to the Gloria and the Credo you can hear that Haydn did indeed set them polytexually. The technique was in widespread use at the time and indeed it would have been surprising if he had not used it. Don't let the fact that this is a short Mass lead you to undervaluing it short it may be but it's a supremely polished piece of music with many highlights. My personal highlights are the wonderfully contemplative opening and closing movements and the glorious solo of the Benedictus but there are many many more facets to this musical gem and no doubt as you listen you'll discover highlights of your own. Enjoy :-).