In Scripture the grace of the Holy Spirit is sometimes called fire, sometimes water, which shews that these words are expressive not of its substance, but of its action. The metaphor of fire conveys the lively and sin-consuming property of grace; that of water the cleansing of the Spirit, and the refreshing of the souls who receive Him.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. xxxii