When was the first medieval church built?

Churches were built prior to medieval times. As to which was oldest, the only "records" we have about a first church are purely legendary. There are several claimants to the honor.

One in particular should be mentioned, which is the construction, prior to 60 AD, of a church in Glastonbury, Somersetshire, which is claimed to have been the first building ever built to be a church. Supposedly it was constructed by Joseph of Arimathea, who had also brought the Holy Grail, to Britain, along with a staff that was planted and became the Glastonbury Thorn. The reason this is a particularly important legend is that papal suits over it were brought at one time or another by kings of both France and Spain, and the suits ended with decisions that the church in Britain was, in fact, the first. (Ever wonder why Arthur's knights were searching for the Grail in Britain.)

Anyway it is a good story, and about as close to the truth as any other.