Jesus urged his disciples to preach to all nations but they never did. The reason they didn't move outside the Roman Empire is because they did not consider the barbarians to be human.
Patrick came back to a country where he had been enslaved. He brought the Gospel to a people who were constantly at war with one another,who were slave traders and were still practising human sacrifice.
Celtic Culture and Beliefs
His years in slavery meant that he had lost out in education and also the social graces. His Latin would always be poor which made it difficult for him to deal with the bureaucratic structures of the Roman Church while his ability to assimilate into Celtic society shows that he had learned at least something of the language and the culture of the country during his enforced stay.
The legend of Patrick using a three leafed shamrock to explain the Holy Trinity displays his understanding of the Celtic fascination for Triads. For instance , Celtic religions accepted that trees and standing stones connected the Underworld , the Living World and the Heavens. This led to the later development of Celtic High Crosses as a means of portraying stories from the Bible.
Influence of Rome
Patrick built on the strong beliefs in nature and creation in existing religious practices and moved away from Roman Christianity. He detested slavery and preached against it although it still existed in the Roman Empire. He supported women in the Church , unlike many of his contemporaries , like Saint Augustine , to whom women represented the temptations of the flesh.
Conversion of A Nation
By restricting himself to Ireland , he managed to convert the entire nation. The tracks of Patrick are to be found everywhere in Ireland to the extent that there are now Saint Patrick's Trails being developed to highlight the main Patrician sites. Considering today's roads and modes of transport , travelling from Armagh, where Patrick established his Episcopal See to Downpatrick where he is buried is even now something of a journey. On foot , in the fifth century, with dangers on every side, it would have been nightmarish.
Yet , this was the man who climbed Croagh Patrick in the West to fast , did penance at Lough Derg in Donegal, baptised Aengus, King of Munster at Cashel and visited with the High King at Tara.
Patrick's impact has since spread throughout Europe in the Dark Ages,when Irish monks carried Irish Christianity back into a world which had suffered following the fall of the Roman Empire.
The missionary zeal of the Irish wherever they have travelled throughout the world since the time of Patrick mirrors the strength and faith of Patrick the missioner who brought the Gospel to Ireland in the 5th century.
Sources:
How the Irish Saved Civilisation by Thomas Cahill Published by Nan A.Talese/Doubleday 1995
Saint Patrick, his Life and Legend by Hector McDonnell Published by Wooden Books 2007
The Saint Patrick You Never Knew by Anita McSorley Saint Anthony Messenger March 1997