Klaas Jan Mulder (1930-2008) was a Dutch organist, pianist, and conductor. As a concert organist, he was performing over 200 concerts per year throughout the Netherlands, France, South Korea, Canada, and the United States. He is also well-known and much loved for his own improvisations, the majority of which are based on hymn tunes and Genevan Psalms. A large number of these improvisations are available to us thanks to the work of organist Marco den Toom, who has been reconstructing and notating them, initially with the help of Mulder before he passed away. This is an ongoing effort, which has thus far resulted in 16 volumes of improvisations published through Marco's "Con Passione" imprint.
The very first piece in the first volume of improvisations is this arrangement of a hymn known better by its German title "Wer nur den lieben Gott lässt walten", or in English, "If thou but suffer God to guide thee". In English hymnals, the tune name is often referred to as NEUMARK.
Sheet music published by Con Passione (CP6004).
https://www.con-passione.nl