On The Love of God :

Love of God is entirely spiritual: "that which is born of the Spirit is spirit" (John 3:6). "That which is born of the flesh is flesh" (John 3:6): carnal love, as something born of flesh and blood, has material, corrupt properties. It is inconstant, changeable: its fire is completely dependent on matter.

Hearing from Scripture that our God is a fire (Heb.12:29), that love is a fire, and feeling in yourself a fire of natural love, do not think that this is one and the same fire. No! These fires are at enmity with one another and are swallowed up by one another (Ladder, steps 3 and 15). "Let us serve in a manner well-pleasing to God, with reverence and fear; for our God is a consuming fire" (Heb. 12:28-29).

Natural love, i.e. fallen love, heats a person's blood, excites his nerves, and arouses daydreaming; holy love cools the blood, calms both soul and body, draws the inner man towards prayerful silence, and immerses him in rapture through humility and spiritual delight. Many ascetics, having taken natural love for Divine love, excited their blood, and excited their daydreams also. The condition of excitement passed very easily into a condition of frenzy. Many took those who were in a state of excitement and frenzy for persons filled with grace and holiness, while they were actually unfortunate victims of self-delusion.

There were many such ascetics in the Western Church from the time it fell into heresy, in which Divine properties are blasphemously ascribed to a man, and veneration which is due and fitting to God alone is given to a man; many of these ascetics wrote books from their excited condition in which frenzied self-delusion seemed to them to be divine love, in which their disordered imagination painted for them a multitude of visions which flattered their self-love and pride.

Bishop Ignatius Brianchaninov (1807-1867)