Do you actually go to church (almost) every Monday? If that's the case, then I would use the Simple Present. If you're just stating that next Monday you will be found there, then the Presetn Continuous is OK.
However, note that you could also use the Continuous for a usual action, but adding something to the sentence:
- How about a beer on Monday evening?
- I'm afraid I can't. On Monday, as usual, I'm going to church. We'll have to leave it for some other time.
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