In 653, Cedd was sent by King Oswiu with three other priests, to evangelise the Middle Angles,[4] who were one of the core ethnic groups of Mercia, based on the mid-Trent valley. Peada, son of Penda was sub-king of the Middle Angles. Peada had agreed to become a Christian in return for the hand of Oswiu's daughter, Alchflaed, in marriage. This was a time of growing Northumbrian power, as Oswiu reunited and consolidated the Northumbrian kingdom after its earlier (641/2) defeat by Penda. Peada travelled to Northumbria to negotiate his marriage and baptism.

Cedd, together with the other priests, Adda, Betti and Diuma, accompanied Peada back to Middle Anglia and won a considerable number of converts of all classes. Bede relates that the pagan Penda did not obstruct preaching even among his subjects in Mercia proper, and portrays him as generally sympathetic to Christianity at this point - a very different view from the general estimate of Penda as a devoted pagan. However, the mission apparently made little headway in the wider Mercian polity, since Bede credits Cedd's brother Chad with the effective evangelization of Mercia, more than a decade later. It seems that, to make progress among the general population, Christianity needed positive royal backing, including grants of land for monasteries, rather than merely a benign attitude.