For the moment let's stipulate that it is circular reasoning; x->y; y->x.  In itself a formal fallacy, this does not negate the fact that the conclusion may still be true.
Now, if the document's early premise is already argued and concluded as true in a different area - that is that the Church is the competent authority on Natural Law - the argument is not circular but follows a typical syllogistic pattern (with the initial premise being understood, not explicit; an enthememe).