"Officiant" works well for a marriage certificate or similar.
An officiant is someone who officiates at (i.e. leads) a service or ceremony, such as marriage, burial, or namegiving/baptism. Officiants may be ordained by any denomination as members of their clergy, or by secular/Humanist or Interfaith/Interspiritual religious bodies. Officiants differ from Chaplains in that Officiants serve the unaffiliated public at large, while Chaplains are usually employed by an institution such as the military, a hospital or other health care facility, etc. The term "Officiant" also includes Justices of the Peace, celebrants, notaries, and other people empowered by law to perform legally-binding private ceremonies.