The Lord’s Supper
What do we mean when we say that the Eucharist is a Spiritual communion with Christ?
The word Spiritual has to do with the mode, not the substance of what we receive. The substance of what we receive is the whole Christ with all of his benefits. The physical body of Christ is life giving to us in the Eucharist, and that is what we receive. This is a mystery, but at the center of the mystery is the work of the Holy Spirit. The Holy Spirit unites us to Christ, and we have true communion with His body and blood. They nourish us unto eternal life. “Christ is not spiritually present as opposed to being bodily present in the sacrament, but gives himself as our food and drink by the agency of the Spirit." 
The bread and the wine in the Eucharist are signs which are never separated from the reality which they signify. The Eucharist is not a “representation” as much as it is a presentation of Christ and his benefits, really and truly. This doesn’t mean that we collapse the sign into the thing signified (as in Transubstantiation), but it also means that we don’t separate them (as Evangelicals so often do). John Calvin wrote,
“I indeed admit that the breaking of bread is a symbol; it is not the thing itself. [i.e. we don’t collapse the two] But, having admitted this, we shall nevertheless duly infer that by the showing of the symbol the thing itself is also shown. For unless a man means to call God a deceiver, he would never dare assert that an empty symbol is set forth by him. [and we don’t sever the two] Therefore, if the Lord truly represents the participation in his body through the breaking of bread, there ought not to be the least doubt that he truly presents and shows his body.” (Institutes IV.XVII.10)
In fact, Calvin could even go on to say that, “…the flesh and blood of Christ are no less truly given to the unworthy than to God’s elect believers” (IV.XVII.33). True, those who partake of the Eucharist without faith receive only condemnation, and not life. But Christ is no more truly presented in the Sacrament to believers than he is to non-believers. Only the faithful truly receive Christ and His benefits, though. St. Augustine (c. 354-430), commenting on “He that eats my flesh and drinks my blood, dwells in me, and I in him” said,
“Consequently, he that dwelleth not in Christ, and in whom Christ dwelleth not, doubtless neither eateth his flesh [spiritually] nor drinketh his blood [although he may press the sacrament of the body and blood of Christ carnally and visibly with his teeth], but rather doth he eat and drink the sacrament of so great a thing to his own judgment…” (On the Gospel of St. John Tractate XXVI.18) 
Therefore, when we say that we have Spiritual communion with Christ, we don’t mean that the substance of our communing is “spiritual” (whatever that means). Nor do we mean that the Holy Spirit is simply representing Christ to us, but that the Holy Spirit is the mystical mode by which we feed on Christ in his whole body and divinity. By the Holy Spirit, we have communion with Jesus Christ who is at the right hand of the Father. Hence Paul could ask, “Is not the cup of blessing which we bless a sharing in the blood of Christ? Is not the bread which we break a sharing in the body of Christ?” (1 Corinthians 10:16) The cup and the bread make us sharers of Christ’s body and blood by the power of the Holy Spirit.
 Michael Horton, People and Place, 128
 imilarly, commenting on John 6:63 Tertullian (220AD†) wrote, “Now, because they thought his discourse was harsh and intolerable, supposing that He had really and literally enjoined on them to eat his flesh, He with the view of ordering the state of salvation as a spiritual thing, set out with the principle, ‘It is the spirit that quickens;’ and then added, ‘The flesh profits nothing,’ – meaning, of course, to the giving of life… Constituting, therefore, His word as the life-giving principle, because that word is spirit and life, He likewise called His flesh by the same appellation; because, too, the Word had become flesh, we ought therefore to desire Him in order that we may have life, and to devour Him with the ear, and to ruminate on Him with the understanding, and to digest Him by faith.” (On the Resurrection of the Flesh XXXVII)