The four great councils of Nicea (325 AD), Constantinople (381), Ephesus (431) and Chalcedon (451) set the parameters within which subsequent theological debate could take place on such issues as the Person of Christ, the Holy Spirit and the Trinity. The written law laid down through the canons and disciplicary decrees defined the boundary between heresy and orthodoxy. Given the great importance of these canons of the ancient ecumenical councils, what precisely do they say and mean? What was the intention of their authors, the fathers of these councils? This work presents authoritative answers to questions such as these.