Which translation of Acts 20:28 is preferable? "Shepherd the church of the Lord, which He purchased with His own blood," or "Shepherd the church of God, which He purchased with the blood of His own Son." Someone told me that the second reading is correct, so that this passage wouldn't show that Christ is also God - a "Trinitarian bias", as he says. I was just wondering if "of the own" (the reading of my interlinear Greek New Testament) was a common phrase in ancient Greek, and if it could mean here that Christ is God, since the Father wouldn't have given His blood for us, and indeed, doesn't have any blood.
The short answer and best translation is "to shepherd the Church of God which He (Jesus) purchased through His own blood (i.e., His death for us on the cross).
If the question then is "who is He/His here", my response would be that, judging from the text, Paul is referring to Christ and the blood of Christ, and that this is neither inconsistent with what he has just said about "the Church of God", nor would Paul have felt there was any problem with putting things this way.
This is really quite a good object lesson passage in a number of ways. It shows, for example, that without some facility in textual criticism, there is no way for laymen (even educated laymen) to tell what the correct reading in a manuscript should be, because there is really no way to garner "perspective" on such things without a lot of experience in both the original language and manuscript issues. Textual criticism is much more of an "art" than a "science" as even the system used by the United Bible Societies text shows (i.e., their "A/B/C/D" level of probability for alternative readings). Moreover, even the "principles of textual criticism" pale in comparison to understanding what an author is saying in the original language (and what he means). For instance, as native English speakers of the American dialect, we can read a student essay and easily see that "I shouldn't of said that" really meant "shouldn't have" - that is, we are familiar with common mistakes, solecisms, idioms, colloquialisms, and know well enough how our language "works" that we can generally pick these out as well as common errors or "typos". This sort of facility is much more difficult to attain in an ancient language no longer spoken, and the issue is complicated by the fact that the various manuscripts were "corrected" in antiquity by people who were "trying to solve problems" which sometimes weren't problems at all. When the Thesaurus Linguae Graecae computer project needed to have all of ancient Greek "data input" some years ago, they farmed it out to a number of overseas concerns. The one with largest error rate by far was in Athens, simply because the modern operators "thought they knew" what the ancient texts should have said (by their erroneous modern mental canons) and therefore consciously or even sometimes subconsciously "made corrections" which were really errors. In addition to this difficulty with ancient manuscripts, we also have the problem of true scribal errors as well (i.e., mistakes in transcription such as leaving words out when one's concentration lapses).
The word idios (the "one's own" you ask about) occurs something like a 100 times in the NT and is a very common Greek word - the English word "idiot" is derived from it (i.e., for the Athenians, someone who was "one's own person" was someone who didn't do their part in civic duties, and that was a big negative). There is absolutely nothing "idiomatic" (another word from the common idios = "one's own way of expression") about the usage in Acts 20:28. The word simply means "[one's] own" as in "belonging to the subject (grammatical or understood)" - that is, just exactly like our English "[one's] own". Whether its his/hers/its/theirs etc. depends on the context, just as in English. So it really is a question of who is the "own" here. In my analysis, it is really of little moment theologically, because the "one's own" does not have to refer to "God/Lord", but only to the "He" who is the subject of "purchased" (and that is clearly our Lord Jesus: 1Pet.1:18-19; Heb.9:12; Rev.5:9; cf. Rom.3:24-25; Eph.1:7; Col.1:14). So "the Church of God, which He [Jesus] purchased with His (i.e., has to be Jesus') own blood" is the best translation/interpretation.
One aside here: it is very, very typical of cults to take verses and words out of context, twisting their meanings, and claiming some "special use" rather than to see passages in the light of the whole language, and in light of the entire epistle/book/Bible - of course, because doing so always explodes their arguments. My own feeling about this passage is that since the best manuscripts have theou ("of God"), this is most likely the original reading. Also, since later readers might not have seen that for Paul the Church is equally of God the Father and of Christ both (they are "one" after all in the sense of undivided purpose in every respect), it might well have been tempting for someone copying a manuscript to assume an error here and to correct it with kyriou ("of the Lord"), the more common and the expected reading. On the other hand, it is hard to see why someone would have emended kyriou with theou, switching thereby to a reading that obviously gives some people pause. We should remember that some theologians who are stuck in traditionalist dogma instead of determined to delve deeper into the Word of God are still arguing about the procession of the Spirit (i.e., "from the Father or the Son?" - as if there could be any difference in agenda amongst the Trinity!).
Some want to read "Lord" here so that the whole predicate will refer to Jesus (so it proves Jesus = God). Others, myself included, would read "God" on the basis of the manuscript evidence, but see no problems with making Jesus the subject of the verb "purchased". Your acquaintance, on the other hand, wants to twist some of the words entirely, no doubt to somehow advance his anti-Trinitarian agenda, hiding behind the general and growing ignorance of Greek. For the one thing that this passage cannot be made to do is refer "His own" here to God the Father to yield the weird translation you have been given above "which He purchased with the blood of His own Son.". The word "Son" just isn't there in the Greek (in any manuscript), and it would have to be there for the passage to bear this meaning.
If we make a point of approaching God's Word on its own terms, and not on the basis of our own preconceived theological frameworks, we inevitably learn something. Here, we learn that our precise separation of Jesus and the Church on the one hand and God (the Father) on the other is apparently an idea that would never have occurred to the apostle Paul. The purposes of the Trinity are so seamlessly interwoven as to be "one" in every way (cf. Jn.10:30: "I and the Father are One"; Deut.6:4: "Hear, O Israel, the Lord, He is our God, the Lord, He is One"). It is true that the Church is usually referred to as belonging to Christ - we are His bride, but we are also "the Church of the Living God" (1Tim.3:15).
What I object to most here is the overall approach being used by your friend. To grow from the Bible, we must respond to it - it is God's Word. When we begin to dictate to the Bible, tell it what it may or may not mean on the basis of preconceived ideas (which may even be 90% correct), we lose an awful lot, not only the chance to be instructed on the particulars we face in any given situation, but also our responsiveness in general. As human beings, we engage in discourse to come to terms with political issues, for example, and that is understandable, since much of the "solution" in the political arena is to be determined by who and what we really are and what we really want and can reasonable be expected to get. Scripture isn't like this at all. Scripture is "God-breathed", and without the breath of the Spirit who gave it in the first place, without a responsive and open attitude to Him, we don't have a prayer of understanding it. That is why the early controversies of the organized church did little beyond establishing what the organized group would not accept (i.e., it was mainly politics imposed on scripture). The legacy of the creeds etc. was to send the organized church on a downward spiral, and burden it with restrictive interpretations (that often do not approach the 90% mark and in many cases close the door on entire areas of truth) under which almost all of the main-line denominations are still suffering today to one degree or another. There will always be heretics who don't care about God. I prefer to cast my lot with those who do want to know Him, genuinely (Ps.16:3). At this late and waning hour in the history of the Church, one would hope that true followers of Jesus would want to begin to devote themselves to finding out in as much detail as possible who and what God is, rather than contenting themselves with who and what He is not. What I mean to say is that the Spirit anxiously waits to show us so much in scripture, and too many of us are myopically focused upon "walling off" what we already think we understand. If we would let Him lead us through the Bible, we would find the brightness and brilliance of Jesus bursting forth into our hearts from every page! For we know of a certainty since we have it from our Lord's own lips that if we ask, we will receive, if we seek, we will find, and if we knock, He will open all things up for us, pouring forth His Spirit bountifully, enlightening our hearts, and leading us into the glorious light of all His truth (Lk.11:9-13).
For more on the "blood of Christ", see the following links:
The Blood of Christ (in BB 4A Christology)
Peter's Epistles #9: Faith and the Blood of Christ
With all who are intent on seeking Him who is the way, the light and the truth, our Lord and Savior Jesus Christ who bought us with His own blood.