To officiate is to carry out an official duty, especially at a ceremony. A priest or pastor officiates at a wedding or a funeral: they lead the service and do the formal parts. A judge or a mayor may officiate at a public event in the same spirit — they are there to do what the role asks. Sports people use it too: the referee officiates the match, meaning they are the official in charge of the rules. The word feels formal. You would not say you officiate at a birthday party unless you were joking. Though actually, the heart of it is “do the duty of the office,” not church alone; church and sport are simply the places learners meet it most.