If Putin was to pull off such an accord, after his conclave of Orthodox churches in Moscow, he could rightfully be declared a “Prince of Christendom”.
While conferring a pseudo-noble title on him, it would also in effect make Moscow an “Orthodox Rome”, religiously akin to the Byzantine Empire in the time prior to Charlemagne, yet one friendly to the Roman Papacy, surviving as it is, surrounded by pagans.
Means something different today.