5th Day within the Octave
Union with God, according to St. John of the Cross. By Fr Gabriel of Mary Magdalen OCD, Chapter II, The presence of God in us (commentary on the 1st stanza in "Spiritual Canticle" by St John of the Cross). 1990 reprint of 1961 edition.

From the first to the last pages of the "Spiritual Canticle", the Saint puts before us the soul anxious for union with God. "Ah, where are you hiding?" it sings in the first stanza of the delightful poem that he will comment on during the whole course of work. It is the soul's cry of desire for union with God. This soul, he notes, is the Christian soul. It is not necessarily a religious soul, a brother, or a nun; it is simply the soul that, regenerated by baptism and reclothed with divine grace, becomes painfully conscious of the potentially contained in its elevation to the state of child of God, and desires to see it brought to fulfillment...Man has need of God. This need finds its first source in our condition as creature. We exist because God created us and keeps us in existence; not only that but we have need of Him continually in order to live and to act. Further, our actions are dependent upon Supreme Being who has to give us capacity for action even in minute things: without Him we cannot move even a finger. There are then so many human undertakings before which the serious and prudent man recognizes more clearly that ever his limitations and the uncertainty of success, which frequently depend on conditions over he cannot exercise his personal influence other than in a very minor capacity....Man has need of God and of having recourse to Him. If nature already orients us toward the Lord, how much more does divine grace. We ought never forget that grace makes us children of God. A man clothed with sanctifying grace belongs to the house of God, he is of the family of God:"You are no longer guests or stranger, but members of God's household."(Eph.2:19)....he who dies in the grace of God will infallibly possess Him eternally in Heaven. Grace therefore disposes us for the beatific vision; it will make us to live in the divine companionship.....That is why there is in the man who lives in the state of grace, a certain inclination, a certain tendency to live in company with God. Unfortunately, there are very many Christians who do not cultivate this beautiful inclination, or who smother it under so many tendencies and natural impulses that draw it toward creatures and distance it from God. When instead a soul seeks to dominate the natural impulses and in this way arrives at a certain interior tranquility, resulting from this domination, or rather from a sufficient mastery of itself, then this inclination toward God, that was as though hidden and buried under the tumult of the passions, awakens and is easily set free. And here it is that the soul begins to feel the need for God, the need to come close to Him, and there come to the lips the words that the Saint puts in the mouth of the enamoured soul: "Ah, where have you hidden yourself?"....Yes, effectively God is in us, even more, He can be there in twofold way: not only with His natural presence because of His immensity, but also with His supernatural (presence) which is called the divine indwelling. Let us explain briefly first the one and then the other.
The first presence of God in us, that which is commonly called the presence of immensity, is a consequence of the creative act of God. We have indicated this above: we exist only through the divine action which communicating being to us preserves us in existence, a communication of which God alone is capable. God does not operate as we do, that is, by means of certain faculties that derive from our essence but are distinct from it. God being simplicity itself, works through his own essence; therefore where He immediately works, there He is. Hence, since He works in the interior of all creatures communicating their being, He dwells necessarily in the inmost part of each of them. Since our soul is a creature, God is necessarily present in us; otherwise, if He were not present, He would not operate in us; nor would He communicate existence to us and we, quite simply, would not "be", as we would not exist. Therefore, with all truth St. John can say to the soul:"You cannot be without Him".
That is not all: close to this fundamental presence, in souls clothed with sanctifying grace there is another that we know only by means of revelation. Jesus has taught us that if a soul loves God supernaturally - and it cannot do so without being in grace - the three Persons of the Most Blessed Trinity will come to it and make their dwelling in it. It is the presence that theologians call "indwelling presence." With this the three divine Persons in a new manner, not simply as creative cause that preserves and moves all things, but as object that offers itself to the knowledge and love of the soul, and therefore as object with the soul can enter into communication. For that reason it is said that with this presence, God comes to keep company with the soul and invites it to keep company with Him.
By this special presence of God in the soul, there is a corresponding capacity in it to put itself in personal relations with Him. That is because where there is grace, there are the three theological virtues, faith, hope and charity, and these supernatural virtues give our soul the capability of establishing an intimate relationship with the Most Holy Trinity. Elevated by faith, our intellect is rendered capable of knowing the Most Holy Trinity who dwells in us; and our will, corroborated by a confident hope and inclined by charity toward God, the author of all supernatural life, can love Him intimately. What more is wanting to us to be able to begin even in this world a relationship and a union of knowledge and of love with that God who dwells in us? It will be enough to put into action our theological virtues; and who can say even where this activity may be able to reach, when to the virtues will be united the gifts of the Holy Spirit which, according to theologians, have the property of rendering our knowledge of God in some ways, at least, experiential. Obviously, here indeed we shall meet contemplation.
The first presence of God in us, that of immensity, is therefore essential here; but much more precious still is the indwelling presence. However, let us not forget this: it is connected with divine grace, and he who loses grace also loses this precious divine company. This is one of the most disastrous consequences of mortal sin: destroying sanctifying grace in us, it also deprives us of the indwelling of the Most Holy Trinity in our souls!