From the Enchiridion of Martin Chemnitz:
For the ancients teach that Christ is united with us not only by the Spirit or by His divinity through faith, but also through that participation which takes place in the Supper in a bodily and natural way and by natural participation. For these are their very words, which speak not only of the mode but of the very nature or substance of the body of Christ.
Some, of course, try to get around these statements of the ancients by saying that Christ is united with us only by some unique spiritual connection which nevertheless is called bodily, that is, true. Others restrict the bodily union only to the incarnation, namely, that Christ assumed, with the exception of sin, the same nature which we possess. Others interpret the bodily or natural participation thus, that the advantages or benefits of the spiritual union consequently apply also to our bodies. Finally, those are most of all pleased with themselves who think they can escape the statements of the ancients by means of the following sophistry. Christ is said to be in us by nature, bodily, and by natural participation because His life, which in the flesh remains very far removed from us, is poured out upon us through the channel of the Spirit. Thus the spiritual properties of which our nature has been deprived because of sin are now by means of the most holy humanity of Christ, although it is very far away, bestowed upon us.
These notions seem plausible and can be embellished by specious arguments. But the fathers stated their position in such ways that they expressly, as it were, rejected such sophistries. For Cyril, In Joannem, Bk. 11, ch. 26, distinctly deals with three modes of our union with Christ:
1. The union by participation through faith with the one Holy Spirit, who by His grace renews our spirit.
2. The union which takes place by reason of our disposition, emotion, or pious attitude or conformity.
The word σχέσις means an affectionate relationship (affectio) of things which are compared with one another, which indicates what agreement one has with another. Thus σχέσις which comes about through love, as Chrysostom and Cyril speak, includes the efficacy and communication of properties. But Cyril has still a third kind of union, distinct from the two previous, namely, that Christ is in us not only with His Spirit and His divinity, not only with the power and efficacy of His flesh, but also with the bodily or natural participation of His flesh. And lest this be restricted only to the incarnation, Cyril says that this participation takes place in the Lord’s Supper (cf. Apology of the Augsburg Confession, Article X – TDD).
Hilary in De Trinitate, Bk. 8 [ch. 13 ff.], speaks still more clearly. He distinctly makes these two points:
1. “We are in Christ through the incarnation because He, being born a man, has inseparably taken to Himself the nature of our flesh, except for sin” (these are his words).
2. Christ is in us by nature, because by that nature which He has assumed from us He has again joined Himself to us by the participation of His flesh in the Supper. Here are his words:
He has added to us the nature of His flesh in addition to the nature of eternity under the Sacrament by the communication of His flesh, and we under the mystery receive the flesh of His body. And thus Christ is in the Father through the nature of the deity, and we are in Him through His physical birth, and He again is in us through the mystery of the sacraments.
This line of argumentation which Hilary uses demonstrates most clearly that these words of his cannot be understood only of symbols or only of efficacy or only of transfusion of properties or conformity of will or consensus of similarity of qualities, on the grounds that Scripture teaches us that the Father is in Christ and Christ is in us. On this basis the Arians used to formulate the following argument. Just as Christ is in us, so the Father is in Christ. But [they said] Christ is not in us with the nature or substance of His flesh but only with His power and efficacy, that is, only with the acquiescence or consensus of His will; therefore the Father is also in Christ not with the very substance or nature of His divinity but only with His power and efficacy.
(Blessed Martin Chemnitz, Ministry, Word, and Sacraments: An Enchiridion, trans. Luther Poellot, eds. Jacob A. O. Preus, Georg Williams; St. Louis: Concordia Pub. House, 2007, 163-165.)