St. Barsanuphius ca. 6th cent.
Concerning warmth and coldness I will say: It is known that the Lord called Himself fire (cf. Deut. 4:25, Heb. 12:29), which warms and kindles the heart and inward parts (cf. Ps. 25:2). If this is so, then the devil, on the contrary, is cold, and from him comes every kind of coldness. If it were otherwise, then why was it said: Then the love of many shall grow cold (Matt. 24:10, 12)? What does “then” mean if it is not the time of the predominance of the adversary? If we feel coldness, let us call upon God, and He will come and warm our hearts with his perfect love not only for Him, but also for our neighbor, and from the face of His warmth the coldness of the hater of good will be banished. (Guidance Toward Spiritual Life, 18)
St. Seraphim of Sarov 1759-1833
God is a fire that warms and kindles the heart and inward parts. And so, if we feel in our hearts coldness, which is from the devil – for the devil is cold – then let us call upon the Lord, and He will come and warm our hearts with perfect love not only for Him, but for our neighbor as well. And from the presence of warmth the coldness of the hater of good will be driven away. (Spiritual Instructions)