Under modern conventions of grammar, titles are uppercase when they refer to a specific person. This is not anything that the Church imposes, but is a convention of (English) language in general. Thus, it is convention to uppercase “Speaker of the House” or “Member of Parliament” when the title refers to a specific person. If the title is used in a generic sense (such as, “the speaker of the house is third-in-line for succession to the presidency”) then lowercase is allowed (but uppercase is not discouraged). We capitalize “Doctor Smith,” but not “see your doctor.”
Such conventions are rather recent - barely a century old.
FWIW, some adopt the convention of uppercasing personal pronouns (he, his) when they refer to God or Jesus. This is more prevalent within protestant styles of writing.
There are no theological “rules” about this whatsoever.