I was there on a Tuesday evening 5-7pm and a few times I looked for a way in but couldn't find it. Not sure ill ever be back in Mainz but still im intrigued.Edited: 4:54 pm, October 30, 2018
And the entrance is from the market square.
thanks for letting me know
How can it close at 5 p.m., and why do you think that's okay? Some people would like to like a candle or say a short prayer after work. (I know of a friend how does.) How can they do that if the Church closes before they even leave the building? Just saying ...
Goodness, Petra... ALL churches have opening hours. Maybe some remote chapel in the countryside does not get locked. In cities they do to prevent vandalism, having homeless sleep in them and what not. I'm sure your pious friend will be familiar with that fact. One doesn't have to go into a church to say a prayer btw.
However, traveler1959 posted the hours of the cathedral's museum. The actual church is open till 18:30 in the summer months except for Saturday when it closes at 16:00. During winter (Nov-Feb) it closes at 17:00 already.