A redated post.
Athanasius had theological reasons for insisting on the doctrine of the Trinity. It wasn't just a matter of "who won the election." He was concerned about what Arianism would do to monotheism (in spite of a unity of purpose between the Father and the Son) and he was also concerned about the fact that if Arianism is true, then someone other than God is saving us. This edition, of course, includes Lewis's fanous preface, otherwise known as "On the Reading of Old Books."