One’s investigation into church history and the Scriptures. Ultimately the same reason anyone affiliates with any given viewpoint.
Well, upon further investigation, let’s investigate the differences of terminology between Christians of different communions. It is certain that such differences form today one of the greatest obstacles at issue. Since the time of the Reformation these differences, which existed even then, have steadily increased, and as long as people fail to realize that they attach different meanings to the same words they will go on talking at cross purposes.
In a certain sense, a common basis is also offered by the first three Creeds, which the reformed communities recognize as norms of belief, namely the Apostles’ Creed, the Nicene, and the so-called Athanasian Creed, which summerizes the whole doctrine of the Trinity and of Christ. We do not ignore the fact that, for example, the meritorious activity of Christ’s human nature is not admitted in Luther’s Christology, but yet the definitive Christology of Chalcedon is fundamentally acknowledged.
In addition, the doctrinal decisions of the first four Councils were recognized by the Reformers and taken as norms of belief. Luther, in his “Concerning Councils and Churches” (1539), was concerned to show how his principle of sola scriptura was consistent with his convinction that the decisions of Nicea (325), Constantinople (381), Ephesus (431) and Chalcedon (451) were norms of belief. His argument was that these Councils simply upheld the teaching of Scripture. This is true enough, and it is also true that, for example, St. Athanasius himself asserted that the Nicene definitions were Scriptural; still the part played by the apostolic tradition in the formulations of these Councils must not be overlooked. It was in light of Tradition that the Fathers of the Councils interpreted Scripture. As is well known, the Arians, Nestorians, and Monophysites appealed to Scripture and adduced a number of texts in their support. The Fathers of the Councils, for their part, to show that their decisions infallibly interpreted the true meaning of Scripture, appealed to the assistance of the Holy Spirit, who preserved them from error as the authorized successors of the Apostles. Luther, however, in reducing the authority of Councils simply to a private judgment which each person must make on the scriptural quality of their decisions, not only disavowed the ancient Church’s idea of Councils, but also overlooked the part played by Tradition in the decisions of the first four of them.