Being an employer and also having been on massive pain meds for an extended period of time I have the following perspective. Tell them ahead of the test. Employer's don't like surprises and you have a perfectly legitimate reason for taking the pain meds. Knowing some of your history, I feel any employer (especially a church!) would be understanding under the circumstances.
1. Yes you did recently have surgery but you came through it well and there is no expectation of any further complications.
2. You have an excellent work record in a very similar job.
3. You have succesfully "weened" off the pain meds.
Quite frankly, this could be considered a plus as opposed to a negative. You have successfully dealt with adversity and you have prevailed. However, if you tried to hide it and get caught the results could be dire.
This is just one guy's opinion, If it doesn't work we are looking for help in my office (can any office stand more than one ostomate? ha! ha!). But I know it's quite a comedown from Sanibel to Westchester County. NY (although it is beautiful here this time of year).
Best of luck whatever you decide.
Ileostomy since August 2005.
As result of surgery (modified whipple) for necrotising pancreatitis. Removal of gallbladder, 2/3 of pancreas, resections of large and small intestines. Massive infection.