A character in 1348 would be speaking Middle English, not modern English at all. Middle English is so different from modern English, that it is a distinct language (source). The same, I suppose, would be true of other European languages as well. So, whatever your 1348 writing would have been, you would have to adapt it, to even make it readable today. Taking a step away from realism is unavoidable here. (Unless your character is a modern person who has studied Middle English, in which case he'd be translating an old writing into modern English, which can be as modern as you like.)
That said, some turns of phrase are more modern than others. "The one who" sounds more modern than "whosoever", for instance. If you want to familiarise youself with that language, you should read older literature: Le Morte d'Arthur, modernised spelling and annotated (otherwise it's not readable), or something else of that period (late 1400s), can get you close enough to the kind of language you seek.