Keep watch over yourselves and all the flock of which the Holy Spirit has made you overseers. Be shepherds of the church of God, which he bought with his own blood—Acts 20:28
What does Paul mean in his speech in Acts 20 when he says that God purchased the church with “his own blood”? God doesn’t have blood. He’s a spirit. In fact, it’s precisely because God doesn’t have blood that God the Son became incarnate. Otherwise the human problem of sin and death could not have been solved. So, what is Paul saying here? Let me try to untangle this one by offering three of the more plausible solutions, one text critical and two interpretive.
A text-critical option. One solution to the problem is found in a handful of important manuscripts that read “church of the Lord” instead of “church of God.” For a list, see the online apparatus of NA28 here. As most recognize, however, the manuscript evidence for this alternative reading is pretty evenly matched with the manuscript evidence for the reading followed above by the NIV. What tips the scales away from this solution, then, is the internal evidence, principally two considerations. First, neither Paul nor any other NT author (incl. Luke) uses the phrase “church of the Lord.” Most often the NT refers to the “church of God” (11x) or to the church of a particular region (“of Galatia”) or city (“of the Thessalonians”). The closest the NT comes to the “church of the Lord” is Paul’s reference in Rom 16:16 to “all the churches of Christ.” Of course, all this could suggest that a scribe changed an original “of the Lord” to “of God” to match the NT’s normal idiom. What points against this, however, is the intrinsic difficulty of the resulting phrase, “the church of God, which he purchased with his own blood.” It seems to me (and others) that a scribe would more likely go against the NT standard idiom than introduce such a difficult theological concept. Thus, “church of God” is the more difficult reading and, as a result, explains the existence of the alternative reading and should be preferred. In other words, the text-critical solution probably won’t work.
Two interpretive options. First, the communicatio idiomatum. This option suggests that Paul uses a quality or property of one of Jesus’ natures—the “blood” of his human nature—to describe or predicate his other nature—his “God”-hood (i.e., God’s blood). In theological discussions, this is known as (one version of) the communicatio idiomatum, the “communication of properties.” This solution to Acts 20:28 has been, as far as I’m able to tell, the standard way of explaining the text throughout Christian history. For a couple high-powered examples, see Calvin’s note here and Jaroslav Pelikan’s note here (pp. 221–22). The trouble with this reading, however, is that it is out of step with the NT. Elsewhere the NT never conflates Jesus’ two natures in this way. While it predicates of the one person what is true of both natures, it stops short of predicating of either nature what is true only of the other (cf., e.g., Harris’ note here). (Luke 1:43 is no exception. On this text, see, e.g., Bock’s comments here.)
Second, a term of endearment. This options suggests that what Paul means here is that God purchased the church with the blood of his Own. That is, “own” refers not to God’s own blood but rather to the blood of God’s Own, which is to say, to Jesus. Thus, the idea would be similar to what we find in, e.g., Eph 1:6 when Paul talks about Jesus as “the beloved” or in Acts 3:14 when Peter calls him “the righteous one.” What points in favor of this option, moreover, is that elsewhere in the NT when “own” is used adjectively (i.e., “God’s own blood”), it’s not often found in the word order used in Acts 20:28. That is—and this one’s for the Greek students out there—it occurs 68x in the first attributive position (art. + adj. + subst.) and only 4x in the second attributive position (art. + subst. + art. + adj.), the position it’s found in here. Added to this, “own” is used substantively in the NT (i.e., “blood of his Own”), and in literature contemporary with the NT, it’s used substantively as a term of endearment (see, e.g., MM here).
While it’s not a total home run, this last option is the best of a bad lot or, as my dad likes to say, it leaves the least number of questions unanswered.