The practice of excommunication represented one of the key points of dispute among various Reformed theories of church discipline. Thomas Erastus represented one end of the spectrum, holding to a decidedly negative view of the practice. The confrontation in the fourth century between Ambrose, the bishop of Milan, and the Emperor Theodosius (depicted in the image above) took on a new significance in light of Erastus' arguments. Noting this incident, Erastus contended that by means of excommunication "it came to pass that the Roman Pontiff subjected to his own power the whole of the West, and compelled princes, kings, and emperors to obey his pleasure." (Erastus, Thomas: The Theses of Erastus Touching Excommunication, trans. Robert Lee, Edinburgh 1844, p. 158).
Anthony van Dyck (1599–1641), St Ambrose barring Theodosius from Milan Cathedral, oil on canvas, 149 x 113.2 cm, ca. 1619–1620; source: © National Gallery, London, http://www.nationalgallery.org.uk/paintings/anthony-van-dyck-st-ambrose-barring-theodosius-from-milan-cathedral.