Hi. "Truth Triumphant" by B G Wilkinson.
An excerpt from the introduction:
.....The doctrines of the primitive Christian church spread to Ireland, Scotland,and Wales. As grains of mustard seed they lodged in the hearts of many godly souls in southern France and northern Italy — people known as the Albigenses and the Waldenses. The faith of Jesus was valiantly upheld by the Church of the East. This term, as used by the author, not only includes the Syrian and Assyrian Churches, but is also the term applied to the development of apostolic Christianity throughout the lands of the East.
The spirit of Christ, burning in the hearts of loyal men who would not
compromise with paganism, sent them forth as missionaries to lands afar. Patrick, Columbanus, Marcos, and a host of others were missionaries to distant lands. They braved the ignorance of the barbarian, the intolerance of the apostate church leaders, and the persecution of the state in order that they might win souls to God.....
This book is available free online to download in PDF form.