Christianity was brought to Britain by the Romans, and when they left, it was preserved in Wales by the monks and reclusive holy men, even under attacks from pagan Anglo-Saxons. 
In the seventh century wandering missionaries came to Wales from Ireland and spread the gospel. They were led by St Patrick (who was Welsh), who had converted Ireland. These Celtic Saints set up centres of learning and prayer. Men like Dyfrig, the first Abbot of Caldey Island off the coast of Pembrokeshire, Deiniol who founded a monastery at Bangor in North Wales, and St Govan’s church built into the cliffs of south Pembrokeshire.
One of the best known was Illtud, who came to be regarded as the ‘most learned of the Britons.’ He set up a monastery at Llantwit Major in the Vale of Glamorgan where hundreds came to study poetry, art, rhetoric and the scriptures. 
The greatest of the Welsh Saints was St David, now the patron saint of Wales. He was born in Pembrokeshire about 520 and established a monastery on the banks of the Alun River. Today St David’s cathedral (above), the smallest cathedral in the British Isles, stands on the site. Most of the stories about David are myths and legends, but he is said to have performed many miracles. The best known is when he preached at Llanddewibrefi and the ground rose beneath him so that the crowd could see and hear him better. He died on 1 March 588, and St David’s Day is celebrated on 1 March every year in Wales.
The influence of the saints can be seen today in Wales in the huge number of placenames which start with Llan. The evangelists created churches surrounded by an earthen embankment. The enclosure was called a llan. Later, this referred to the church itself, with the name of its founder, and gave its name to the settlement around the llan. Place names such as Llandeilo (Teilo’s llan) and Llanilltud (Illtud’s llan).
Note: The double L sound in Welsh is made by putting the tip of the tongue behind the top teeth, as if you were going to say L and blowing. You can find a simple guide to welsh pronounciation here.
[Adapted from Highlights of Welsh History by Phil Carradice]