It cannot be over stressed that the presence of God in the soul by grace is a real and substantial presence. God is present in the tabernacle of the heart as really and truly and substantially as He is present in the tabernacle of the altar, although in a different manner.
Christ’s Real Presence in the Blessed Sacrament is a priceless gift, a pledge and prelude of that glorious presence of the Incarnate Word which will be ours eternally in Heaven. But the external presence is meant to lead us to and be fused with the presence of the Word of God within our souls. So it will be in Heaven. The Son of God will be present outside us in the reality of the human nature He has assumed, the peak and glory of creation; yet that same Son of God will dwell substantially within us according to His divine nature. One presence is not opposed to the other, but complements it. That will be self-evident in Heaven. But here too devotion to the Incarnate Word present on the altar is not a hindrance to devotion to the Word of God within me; quite the reverse. All Christian experience goes to show that it nourishes it as nothing else can do. On the other hand, devotion to the Second Divine Person within me will urge me to seek Him also in the human nature which makes Him my brother and which faith tells me is truly present in the tabernacle.
There is a point, however, worth bearing in mind. If I personally fall to honor Our Lord in the Blessed Sacrament, He will not remain without honor there. Other faithful Catholics will give Him the honor that is due to His presence and so compensate to some extent for my negligence. But if I fall to honor the Son of God as He is present with the Father and Holy Spirit in the tabernacle of my own heart, no man can compensate for my indifference. The inner sanctuary of my own heart is utterly and eternally inaccessible to any except myself and God. If I am in the state of grace, God is really present there in most profound silence and solitude. None but myself can hope to penetrate into that inmost sanctuary to do Him honor and hold converse with Him. It is quite possible that in fact I have never penetrated there. There are many people—dare we say most people?—who have never withdrawn themselves from the distractions of the senses to enter into that sacred shrine of the soul where they can be alone with God. Yet without devotion to the real presence of God within our souls there can be no full development of our spiritual life, which is essentially an interior life. “Let it be plainly understood that we cannot return to God unless we first enter into ourselves. God is everywhere, but not everywhere to us. There is but one point in the universe where God communicates with us, and that is the center of our own soul. There He waits for us; there He meets us; there He speaks to us. To seek Him, therefore, we must enter into our own interior” (Archbishop Ullathorne).
(Father Anselm Moynihan, O.P. from The Presence of God)