After googling it came out that it was indeed a venue for christian service, but it is no church itself. The actual name is "Kleines Waldforum" (see  https://www.abendblatt.de/hamburg/article234145939/beliebtem-hamburger-ausflugsziel-droht-die-schliessung-wildgehege-kloevensteen-altona-tiere.html), and it's a venue for several events. In my opinion "amenity=theatre" and "theatre:type = open_air" already match okayish (but way better than church). But maybe there is a even better fit?