Wachet auf, ruft uns die Stimme

Gustav Adolf Thomas (1842-1870) was a German organist, music editor and composer.
That's all I could find on him.

Anyone, who can provide some more information:
Herzlich willkommen!

This is a typical example of the way church services would begin and end in Germany in the 19th century: with a fantasy or "nachspiel" on a well-known hymn melody.
Lots of organists wrote their own pieces, which were usually meant to be played on the full organ.
There is quite good craftmanship in most of them, easy to listen and written by fine musicians.
And now and then one sparkles between the smaller stars....