“Ask, O ye Donatists, if ye know it not, ask how many stopping-places there were in the Apostle’s journeys round about unto Illyricum.”

A.C. Headlam regards these words as “the argument of the whole treatise” http://books.google.com/books?id=gxjlXxw0HMMC&q=%22This+is+the+argument+of+the+whole+treatise.%22&dq=%22This+is+the+argument+of+the+whole+treatise.%22&hl=en&ei=UWdQTovZBqrJsQKNs73JDQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCoQ6AEwAA See, The Doctrine of the Church and Christian Reunion: Being the Bampton Lectures for the Year 1920 http://www.archive.org/details/doctrineofchurch00headuoft, Rev. Arthur Cayley Headlam, D.D., London, John Murray, p. 152.
The Gospel and Catholic Church, (1936, reissue ed. 2009) http://books.google.com/books?id=RACb6TICT4QC&pg=PA131&dq=%22ask+o+ye+donatists%22&hl=en&ei=NpJRTt2qD8rlsQKsnvzQBQ&sa=X&oi=book_result&ct=result&resnum=1&ved=0CCoQ6AEwAA#v=onepage&q=%22ask%20o%20ye%20donatists%22&f=false, Michael Ramsey, Hendrickson Publishers; , p. 131
De Unitate Ecclesiae - On the Unity of the Church (c. 401 – 405)
Context: Ask, O ye Donatists, if ye know it not, ask how many stopping-places there were in the Apostle’s journeys round about unto Illyricum. Add up the number of the churches, and tell me how they have perished through our African strife. Corinth, Ephesus, Philippi, Thessalonica, Collosae - you have only the letters of the Apostles to read which he addressed to them. We read the letters, we preserve the faith. We are in communion with the churches. (ch.12:31)