Thank you letters, for a church service or any other activity, should be brief, sincere, personal and precise, specifying to whom you are giving thanks and the event or effort for which you are thanking them. It is important that your thanks be conveyed as soon as possible after the event in question.
Etiquette guide Olivia Palermo states that the presentation of the note means as much as the content of the note. When thanking a church officiant, choose an appropriate card or use fine stationery, preferably with a religious or spiritual motif. Hand write the note, taking time to keep the penmanship neat and to avoid spelling errors. If need be, practice on scrap paper before writing the final draft.
Address the officiant by his or her official title. Thank him or her for the church service, and be sure to specify what the service was for. State briefly a specific action or a specific moment in the service that was especially touching or memorable, and state unambiguously that his or her actions were meaningful and touching. Close with a final "thank you."
Write the note as soon as possible after the event, and deliver it to the officiant as quickly as is feasible.