The following is from Saint Diadochos of Photiki's work On Spiritual Knowledge and Discrimination: One Hundred Texts.
He who loves God both believes truly and performs the works of faith reverently. But he who only believes and does not love, lacks even the faith he thinks he has; for he believes merely with a certain superficiality of intellect and is not energized by the full force of love's glory. The chief part of virtue, then, is faith energized by love.
HT: Tony-Allen, where you can read the larger passage.