Letras can be lyrics of a song. If they are referring to a hymn or possibly a chant lyrics would apply.
Otherwise I would stay more general and use wording.

Literally it is of course "letters" but your translation should express the meaning in context. If this is a formal and solemn document within a church then the "letras" may be an expression of solemnity.

I would not translate it "letter" unless it really is intended as a letter. The use of patriarchal suggests that the model is a formal document of bishops or the equivalent to a church.

Perhaps the sense is "message."

In short, move beyond the literal meaning to the meaning in context.