It is not certain when Christianity came to Wales. Although there are legends of two men being martyred for their faith in Caerleon, and archaeological evidence of a Christian chapel being laid over a mosaic floor in a Roman villa, the actual date remains elusive. We can however say with some certainty that it was after the conversion of Constantine that it occurred. We can also say that Wales and therefore Anglesey was Christianized by Celtic saints in a unique period known as the Age of Saints.
According to the late Professor E G Bowen in "Saints, Seaways and Settlements" Celtic Christianity spread along the Western seaways when the Anglo Saxon invasion made the eastern routes unusable. Thus it was that Christian doctrines could find their way to Ireland, via Spain, Portugal and Brittany, and thence to Wales, Cornwall, and Scotland. Evidence of this Irish influx is symbolized by the story of Brychan Brycheiniog, erstwhile king of the Welsh county that bears his name. According to a document called the "De Situ Breceniauc" (12th Century) Marchell, Brychan’s mother, travelled from the area of what is now Breconshire to marry Amlach an Irish prince. It is symbolic of the traffic between Ireland and Wales in the early post Roman Period, where travellers and settlers used the Roman roads to gain the coast. Testimony to this Irish influx is the presence of stones inscribed with the Ogham script, which originated in Ireland.
Brychan himself appears to have been an early ruler of Breconshire, and is credited with an impossible number of children, all of whom became saints. In Anglesey the Brychan dynasty is represented by St Dwynwen (Dwyn) of Llanddwyn, who became the Welsh patron saint of lovers, St Ceinwen (Cain or Kein) of Llangeinwen, St Dyfnan of Llanddyfnan, and St Caian of Tregaian or Tregeuan. Although the story of Brychan and his family has not been fully analysed, it seems that they represent some of the earliest evangelising movements in Wales and therefore Anglesey. The date of 465 on the Latin cross commemorating Dwynwen on Llanddwyn may not be too wide of the mark.
Later saints were St Cybi of Caergybi (Holyhead) and St Seiriol of Ynys Seiriol and Penmon. These two met at the wells of Clorach near Llanerchymedd, and were sometimes joined by St Eilian of Llaneilian. The purpose of the meetings was to discuss the religious matters of the day. The legend is that Cybi, travelling from the west, faced the sun on both legs of the journey, and thus was called Cybi Felyn (yellow or tanned Cybi - see poem). Seiriol on the other hand travelling from the east had his back to the rising and setting sun, and thus was called Seiriol Wyn (white Seiriol).
Another famous saint had establishments in Anglesey. This was St Beuno of Clynnog, the uncle and resurrector of St Winifred of Holywell. He has a foundation at Aberffraw. Other saints are less well known, such as Peirio of Rhosbeirio, and Ewryd of Bodewryd. Peirio is often recognised as Piro (Sabine Baring-Gould and John Fisher - The Lives of the British Saints), but their histories are very obscure. Churches bearing non-Celtic names are either very late or are foundations on abbey land, most notably Aberconwy. There are churches dedicated to St Mary (Mair), St Peter (Pedr), and St Michael (Mihangel), as in Llanfairynghornwy, Llanbedrgoch, and Llanfihangel-Tre'r-Beirdd. (Full listing of Anglesey Dedications. Full listing of Anglesey Churches)
The Age of Saints as it is known is part of a very dynamic period in Welsh, and therefore of Anglesey history. This period became a sort of melting pot, into which went a post Roman pagan Celtic country, with its Brythonic or British language, and out of which there emerged a Christian, Welsh Nation with its distinctive language.
In Llangaffo Church there is a collection of decorated and inscribed stones dating from the 7th to the 13th Centuries. The names Curris, Cinnin, Virnen appear. Because the names bear no gender endings (as they would in the ancient British language) they are considered to be some of the earliest words written in Welsh. The late Professor of Welsh at Bangor University Melville Richards urged all true Welsh people to make a pilgrimage to this site.