St. Symeon the New Theologian: God is fire and when He came into the world...

God is fire and when He came into the world, and became man, He sent fire on the earth, as He Himself says; this fire turns about searching to find material — that is a disposition and an intention that is good — to fall into and to kindle; and for those in whom this fire will ignite, it becomes a great flame, which reaches Heaven. … 

[T]his flame at first purifies us from the pollution of passions and then it becomes in us food and drink and light and joy, and renders us light ourselves because we participate in His light.

St. Symeon the New Theologian