The name Devizes is thought to be a corruption of the Latin "ad divisas", meaning 'at the divisions or boundaries'.
The boundaries were those of the three adjoining manors, Rowde, Cannings and Potterne, which met precisely at the point where Devizes Castle, built by Osmund, Bishop of Salisbury in about 1080, was constructed.
The Churches of Devizes St. John and St. Mary have an inextricably linked history, as it is reported that they have always been "a single cure" under a single Rector, rector ecclesiarum loci, as he was called in 1322. This however did not prevent the two churches from having separate incomes and separate parish officers.
There was a move made in 1906 by the Rector, J. G. Watson, to separate the parish of St Mary but this was not successful.
It has always been presumed that St John's was originally the castle chapel, and "in very early times the garrison may have been large enough to fill it", and that St Mary's served the growing civilian settlement. But as there was a large chapel in the castle it is probable that both St John's and St Mary's were built as a pair in the period 1120-1135 by Bishop Roger to serve the civilian populations in both parts of the town.
More information about the history of Devizes can be found at the Devizes Heritage Website.