…to shepherd the church of God, which he purchased with his own blood.

But because the speech which Paul uses seems to be somewhat hard, we must see in what sense he says that God purchased the Church with his blood. For nothing is more absurd than to feign or imagine God to be mortal or to have a body. But in this speech he commends the unity of person in Christ; for because there be distinct natures in Christ, the Scripture does sometimes recite that apart by itself which is proper to either. But when it sets God before us made manifest in the flesh, it does not separate the human nature from the Deity. Notwithstanding, because again two natures are so united in Christ, that they make one person, that is improperly translated sometimes unto the one, which does truly and indeed belong to the other, as in this place Paul attributes blood to God; because the man Jesus Christ, who shed his blood for us, was also God. This manner of speaking is called, of the old writers, communicatio idiomatum, because the property of the one nature is applied to the other. And I said that by this means is manifestly expressed one person of Christ, lest we imagine him to be double, which Nestorius did in times past attempt; and yet for all this we must not imagine a confusion of the two natures which Eutychus went about to bring in, or which the Spanish dog, Servetus, has at this time invented, who makes the Deity of Christ nothing else but a form or image of the human nature, which he dreams to have always shined in God.

—John Calvin, Commentary upon the Acts of the Apostles, trans. Henry Beveridge (repr. Bellingham, WA: Logos Bible Software, 2010), 2.256–57. Vocabulary and spelling modernized.

…But the communicating of characteristics or properties [communicatio autem idiomatum sive proprietatum] consists in what Paul says: “God purchased the church with his blood” [Acts 20:28], and “the Lord of glory was crucified” [1 Cor. 2:8]. John says the same: “The Word of life was handled” [1 John 1:1]. Surely God does not have blood, does not suffer [nec patitur], cannot be touched with hands. But since Christ, who was true God and also true man, was crucified and shed his blood for us, the things that he carried out in his human nature are transferred improperly [improprie], although not without reason, to his divinity. Here is a similar example: John teaches “that God laid down his life for us” [1 John 3:16]. Accordingly, there also a property of humanity is shared with the other nature. Again, when Christ, still living on earth, said: “No one has ascended into heaven but the Son of man who was in heaven” [John 3:13], surely then, as man, in the flesh that he had taken upon himself, he was not in heaven. But because the selfsame one was both God and man, for the sake of the union of both natures he gave to the one what belonged to the other

—John Calvin, Institutes of the Christian Religion trans. Ford Lewis Battles, The Library of Christian Classics (Louisville, KY: Westminster John Knox Press, 2011), 2.14.2.