As the heart settles more and more into stillness, the fire of divine love begins to emerge. This fire is none other than the divine indwelling of the Holy Trinity. This presence is given through the sacraments but emerges and takes on its expression in the lives of believers as the layers of sin and attachments begin to fade away. As the believer develops the interior silence and receptivity to this divine indwelling, the heart begins to feel the energy and warmth of divine love pervading all of created reality.
This language is, of course, analogous. Fire, warmth, and other uses of language are meant to approximate a whole series of spiritual insights and experiences which are not properly speaking sense experiences. The saints use such images and language as means to describe what their experience is like while realizing the limitations of such language. An awareness of the divine indwelling feels like a fire, and the bursting forth of joy and contentment feels like warmth.
Spiritual experience has an ineffable quality to it. St. John of the Cross called the knowledge gained from the Spirit as dark and obscure to highlight this point. By this he means that the knowledge gained through spiritual experience is not discursive. It cannot necessarily be proven or explained in way that someone who has not shared in these experiences will immediately understand. It can, however, be shared among those along the path by certain approximations. Other writers would talk about the spiritual senses whereby spiritual experiences were related to the different physical senses. For example, we talk about faith as a kind of sight, whereas in reality one still sees the same created reality as before.
As useful as analogy might be, spiritual writers also use paradoxes as a means of describing their insights. In this sense, the stillness of a heart that lives in conformity with Christ can also be said to moved and alive with a dynamic energy that compels the believer to seek the Lord with unlimited desire. Thus the heart is both still and not-still; at rest but yet restless; desiring nothing but God, but yet filled with a kind of craving desire for God’s will.