Here's a short story:

Westminster Abbey, formally titled the Collegiate Church of St Peter at Westminster, is a large, mainly Gothic abbey church in the City of Westminster, London, located just to the west of the Palace of Westminster. It is one of the most notable religious buildings in the United Kingdom and has been the traditional place of coronation and burial site for English and, later, British monarchs. Between 1540 and 1556 the abbey had the status of a cathedral. Since 1560, however, the building is no longer an abbey nor a cathedral, having instead the status of a Church of England "Royal Peculiar"—a church responsible directly to the sovereign. The building itself is the original abbey church. 

According to a tradition first reported by Sulcard in about 1080, a church was founded at the site (then known as Thorn Ey (Thorn Island)) in the 7th century, at the time of Mellitus, a Bishop of London. Construction of the present church began in 1245, on the orders of King Henry III. (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Question: When was a church first founded on the site of Westminister Abbey?

7th century