Alfred the Great (9th century king of Wessex) was a huge proponent of the vernacular as an easy option for a country which he saw as being so illiterate that they’d displeased God, to the extent that it had caused the Viking invasions. He personally translated some works into English vernacular to make them more accessible, in the hope that soon all the sons of earls etc could be literate - this includes the Pastoral Care of Gregory the Great, to which he adds a prologue about his motives. He also began a court school, which taught both English and Latin, to improve literacy. Before Alfred, our best examples of ‘English’ grammar are shoddy and poorly written, but from the 9th century onwards we really see an improvement in written English, as opposed to Latin, partially through his influence.