I found this sign a little odd - it means the church is open. Even taken literally, I wouldn't think you would need a sign to tell people the church is open, but perhaps it is meant in a more metaphorical sense? Or maybe things are just different here!
In any case, I did take advantage of the offer to come inside the Gethsemanekirche and found it very much worth my time. The interior is quite simple, but the really interesting part of the church is that it played a very important role in the fall of the Berlin wall. Although religion was generally frowned upon in the former East, it was to some degree tolerated. In the late 1980s, young people began organizing prayer services and rallies at this church. Sometimes so many people came to pray that they were overflowing into the street. I guess the power of prayer really did work in this case as the wall eventually came down. There is a small exhibit inside the church detailing its role in the events of Autumn 1989 which is definitely worth a visit if you're in the area. Until then, you can see more signs at Signs, Signs.