Johann Kuhnau was Bach's predecessor as cantor in the Thomaskirche in Leipzig. He composed beautiful cantatas and keyboard music, under which a collection of six Biblical Sonatas. That genre was quite unique, especially for the period of the German baroque he lived in.
Some of these sonatas (especially the most famous one of the battle between David and Goliath) do sound best on the harpsichord. Some of them do sound quite nice on organ too, like the 4th Sonata about the sick and recovering Hezekiah.
The explanation of what it is about is originally in Italian. I've used a translation in English, so we all can understand it. In the video I've highligthed the description which fits the part you listen to.
The sonata consists of three parts:
I. Hezekiah's lament and prayers
II. Hezekiah's faith in God
III. The joy of the recovering King