How, you ask, would the Christian tradition defend itself against the Islamic reproach that the Incarnation (hulul) “restricts” the Divine Presence? You would like to know, more precisely, what a Christian might say to those “several Sufis” who claim—citing the Koranic text, “Wheresoever ye turn, there is the Face of God” (2:115)—that “Islam is superior to Christianity because it acknowledges the immanence of God as being everywhere present and not confined to Christ alone”.
Obviously one could say a great deal!—more than I can possibly manage in this forum. What I can tell you briefly, however, is that no thinking Christian has ever believed that in becoming incarnate God was somehow obliged to fit Himself into the space of Christ’s body or that henceforth He could be found nowhere else. Be assured that Christian theologians, no less than their Muslim counterparts, understand perfectly well that the Divine omnipresence, unlike the mere ubiquity of something like space, cannot be divided up into parts or sectors. Though it is true that in Christ “all the fullness of the Godhead was pleased to dwell” (Col. 1:19), the Supremely Real remained everywhere else in its fullness as well.
Perhaps in lieu of the book one would need to write on this subject, I can at least point you in the right direction by citing three crucial texts, two Patristic and one Biblical. God willing, you can then triangulate to the point of glimpsing a more thoroughgoing and appropriately nuanced answer for yourself. You might also wish to look back at my earlier posts on this weblog concerning the hypostatic identity of Christ as the Logos: notably, “Enhypostatic Humanity“, “Asymmetrical Dyophysitism“, “Occupying the Same Space“, and “No Longer Dying Death as Before“.
The first text I have in mind comes from the Quicunque Vult, or “Athanasian Creed” (c. 500 A.D.): “God became man, not by the conversion of the Godhead into flesh, but by the taking of the manhood into God.” As you can see from this passage, the Incarnation is not to be understood as a Divine diminishment but rather as a human expansion or liberation, God’s in-carnation entailing man’s ex-carnation. It is thus that Saint Paul speaks of being “delivered from this body of death” (Rom. 7:24)—though let us hastily add that his words in no way conflict with traditional Christian (and Muslim) doctrine concerning the Resurrection of the body, albeit a “spiritual body” (1 Cor. 15:44).
A second key passage is found in the writings of Saint Maximos the Confessor: “Always and in everything God the Logos seeks to work the mystery of His Incarnation” (Ambigua, PG 91, 1084D). The saint calls our attention specifically to two other forms or modes of Divine “embodiment” (though without implying they exhaust the possibilities): namely, the Cosmos and the Scriptures. In any case it should be crystal clear from this text that the immanence of God in Christ, far from being in competition with other modes of manifestation, allows—indeed obliges—the Christian just as much as the Sufi to “acknowledge the immanence of God as being everywhere present”.
Of course, Christians are going to insist that God’s incarnate manifestation in Christ, while not exclusive, is nonetheless decisive. But if one grasps—or rather admits he can’t grasp!—the paradoxical “position” of this Christic presence, I don’t see how one could ever suppose their insistence means a limitation on God. You’ll understand what I’m talking about if you take a look at a third text, this one from the Gospel of John, where Christ is speaking to Nicodemus: “No man hath ascended up to Heaven but He that came down from Heaven, even the Son of Man, which is in Heaven” (John 3:13). In other words, nobody goes up except the One who came down, and though He certainly is “down”—otherwise He wouldn’t be standing right there in front of the poor flummoxed Pharisee—He’s at the same time still “up”!
What would a Christian say to a Sufi who was concerned that the incarnation places a restriction on God? I think he might quote the Shaykh al-Akbar: “Do not declare Him non-delimited and thus delimited by being distinguished from delimitation! For if He is distinguished, then He is delimited by His non-delimitation. And if He is delimited by His non-delimitation, then He is not He” (Futūhāt al-makkiyya; see William Chittick, The Sufi Path of Knowledge: Ibn al-Arabi’s Metaphysics of Imagination [SUNY, 1989], p. 112).