Yeats himself, religious by temperament but unable to believe in Christian orthodoxy, sought all his life for traditions of esoteric thought that would compensate for a lost religion. This search led him to various kinds of mysticism, to folklore, theosophy, spiritualism, and neo Platonism - not in any strict chronological order. He kept reworking earlier aspects of his own thinking.
In midlife he elaborated a symbolic system of his own, based on a variety of sources. This enabled him to strengthen the pattern and coherence of his poetic imagery.
His greatness as a poet lies in his ability to communicate the power and significance of his symbols, by the way he expresses and organizes them, even to readers who know nothing of his system.
Page 1859 Volume 2 Norton anthology