How much of Britain had Christianity reached in AD 300?

Answer 1

The North African theologian, Tertullian, writing ca. AD 200, said that the living places of Britons (one of the tribes inhabiting Britain) were "subjugated to Christ," even though these places were inaccessible to the Romans. It suggests that Christianity was known throughout Britain, but there is hyperbole here. It would have sufficient for there to be a few Christians here and there amongst the Britons, and that seems quite likely.

Archaeological evidence suggests Christian communities were established in southern Britain and as far north as modern Lincolnshire around AD 300.

Additional Answer:

There are writings from the Greeks that have the Apostles going to Britian in the middle of the 1st Century. Here is one example:

Metapirastes, the Greek historian, reports, "that Peter was not only in these WESTERN parts"-the Western Mediterranean-

"But particularly that he was a long time"-here we have Peter's main life work to the Lost Ten Tribes-"... a long time in BRITAIN, where he converted many nations to the faith." (See marginal note, p. 45, in Cave's "Antiquitates Apostolicae").

Then there was Simon the zealot. Simon "directed his journey toward Egypt, then to Cyrene, and Africa ... and throughout Mauritania and all Libya, preaching the gospel.... Nor could the coldness of the climate benumb his zeal, or hinder him from whipping himself and the Christian doctrine over to the WESTERN Islands, yea, even to Britain itself. Here he preached and wrought many miracles...." Nicephorus and Dorotheus both wrote, "that he went at last into BRITAIN, and ... was crucified ... and buried there" (p. 203 of Cave's "Antiq. Apost.").

The Apostle James book is addressed to the 12 tribes of Israel, apparently warring at the time of this writing in circa 60 A.D. Could this be the wars in Britain at that time. Afterall, the Apostles were tasked with preaching the Gospel to the lost tribes too. Some think so as about A.D. 60 great wars overtook Britain just as James warned (James 4:1) the twelve tribes of Israel. Historians are hard pressed to find another major war happening at this time.

For the Apostles to complete their task of preaching the Good News to all in the House of Israel, and history shows their travels to Britain, Scotland and Ireland, it would seem to me that they reached all or planted seeds to reach all in these isles.