| | Re: Help with "May God have mercy on your soul"
How about 'May God have mercy on his soul.' or 'Let God have mercy on your soul.' or 'Let God have mercy on his soul.'?
The difference between 'let' and 'may' must have something to do with what Michael Swan (and others?) calls interpersonal uses:
'may' would be one such 'interpersonal' verb (1st person wishing 2nd person sth)
whereas 'let' would not (1st person wishing 3rd person sth).