He left behind a large body of spiritual and theological writings. Selections from his works make up the largest single part of the Philokalia. A short but interesting sample is reproduced below. It discusses the spiritual meaning of various numbers associated with the person of Jesus. Numerology frequently occurs in ancient and patristic writers and goes back at least as far as Pythagoras.
The excerpt is from vol 2 of The Philokalia (tr and ed by GEH Palmer, Philip Sherrard, and Kallistos Ware), p 130.
The Lord appeared when He was thirty years old, and with this number secretly teaches those with discernment the mysteries relating to Himself. For, mystically understood, the number thirty presents the Lord as the Creator and provident ruler of time, nature, and the intelligible realities that lie beyond visible nature. The number seven signifies that He is the Creator of time, for time has a sevenfold character. The number five signifies that He is the Creator of nature, for nature has a fivefold character because of the fivefold division of the senses. The number eight signifies that He is the Creator of intelligible realities, for intelligible realities come into being outside the cycle that is measured by time. And the number ten signifies that He is the provident ruler, because it is the ten holy commandments that lead men towards perfection, and also because the symbol for ten [in Greek] is the first letter of the name taken by the Lord when He became man. By adding up five, seven, eight and ten you obtain the number thirty. Thus he who truly knows how to follow the Lord as his master will understand why, should he attain the age of thirty, he will also be empowered to proclaim the gospel of the kingdom. For when through his ascetic practice he has irreproachably created the world of the virtues as if it were a world of visible nature, not allowing his soul to be diverted from its course by the hostile powers as he passes through time; and when he unerringly gathers spiritual knowledge through contemplation, and is providentially able to engender the same state in others, then he himself, whatever his physical age, is thirty years old in spirit and makes manifest in others the power of the blessings which he himself possesses.