St. John of the Cross on Perfect Transformation in God

Since every living being lives by its operations, as the philosophers say, and the soul's operations are in God though its union with him, it lives the life of God. Thus it changed its death to life, its animal life to spiritual life.

The intellect, which before this union understood naturally by the vigor of its natural light by means of the natural senses, is now moved and informed by another higher principle of supernatural divine light, and the senses are bypassed. Accordingly, the intellect becomes divine, because through its union with God's intellect both become one.

And the will, which previously loved in a base and deadly way with only its natural affection, is now changed into the life of divine love, for it loves in a lofty way with divine affection, moved by the strength of the Holy Spirit in which it now lives the life of love. By means of this union God's will and the soul's will are now one. 

And the memory, which by itself perceived only the figures and phantasms of creatures, is changed through this union so as to have in its mind the eternal years mentioned by David [Ps. 77:5].

And the natural appetite that only had the ability and strength to relish creatures (which causes death), is changed now so that its taste and savor are divine, and it is moved and satisfied by another principle: the delight of God, in which it is more alive. And because it is united with him, it is no longer anything else than the appetite of God.

Finally all the movements, operations, and inclinations the soul had previously from the principle and strength of its natural life are now in this union dead to what they formerly were, changed into divine movements, and alive to God. For the soul, like a true daughter of God, is moved in all by the Spirit of God, as St. Paul teaches in saying that those who are moved by the Spirit of God are children of God himself [Rom. 8:14].

Accordingly, the intellect of this soul is God's intellect; its will is God's will; its memory is the memory of God; and its delight is God's delight; and although the substance of this soul is not the substance of God, since it cannot undergo a substantial conversion into him, it has become God through participation in God, being united to and absorbed in him, as it is in this state. Such a union is wrought in this perfect state of the spiritual life, yet not as perfectly as in the next life. Consequently the soul is dead to all it was in itself, which was death to it, and alive to what God is in himself. Speaking of itself, the soul declares in this verse: "In killing you changed death to life." 

The soul can well repeat the words of St. Paul: I live, now not I, but Christ lives in me [Gal. 2:20]. The death of this soul is changed to the life of God. 

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Source: St. John of the Cross, The Living Flame of Love, in The Collected Works of St. John of the Cross, trans. by K. Kavanaugh and O. Rodriguez (Washington, DC: ICS Publications, 1991), 2.34.