Bill Mounce, ESV translator, is right on target:
I have always taught that Greek grammar doesn’t necessarily answer all the questions definitively. Sometimes it does, but normally it gives us the legitimate range of possible interpretations, and then context and theology make the final determination.
Read Mounce’s whole post.
One of the most helpful things I learned from my doctoral courses at BJ Seminary—and I learned this especially from Dr. Randy Leedy of BibleWorks NT diagram fame—is that Koine Greek isn’t some unique Holy Ghost language with specialized word meanings and hyper-precise grammatical constructions. No, God chose a normal human language to communicate His truth. Putting a lot of theological weight on one word—whether a supposed technical term or just a preposition—is generally a violation of the way normal human language works. Like Mounce says, “context and theology make the final determination.”