The apostle Paul could at times summon quite shocking imagery to jolt his readers into appreciating his points. One common example is his response to the Judaizers who were upsetting the churches of Galatia (Gal 5:12) in which he wishes that those who taught the importance of circumcision would end up as eunuchs. Another common example is the way he evaluated his "credentials" in his letter to the Philippians as "rubbish" (Phil. 3:8), which is quite a toned-down translation of a word that meant either "dung" or "foul-smelling street garbage." "It is hard to imagine a more pejorative epithet than this one..." 
A third example comes from our text this week. As Paul stands before the Sanhedrin, he begins by appealing to a clear conscience before God in all his actions, which so provoked the high priest that he ordered those who were standing around Paul to smack him in the mouth. Then, Paul responds with a jolting retort: “God is going to strike you, you whitewashed wall! Are you sitting to judge me according to the law, and yet contrary to the law you order me to be struck?" (Acts 23:3 ESV). When the bystanders completely ignore the high priest’s unjust actions and criticize Paul, he ends the exchange with a rather strange “apology” if it actually was one: "I did not know, brothers, that he was the high priest, for it is written, 'You shall not speak evil of a ruler of your people'" (Act 23:5 ESV).
My hope with bringing this up is to briefly reflect on the use of strong language in the Scriptures. With that in mind, we should note that Paul appears almost prophetic in his response because he uses the high priest’s action as the occasion for delivering a curse. As Paul was struck, so God would strike the high priest. But this wasn’t a vindictive response. He continues by asking a rhetorical question that highlights the hypocrisy of the high priest. The very one who is supposed to be judging Paul by the measure of the law can’t even follow that law himself. If you’re hearing echoes of Paul’s statements in some of his letters, I’m hearing them, too.
But I forgot my favorite part of Paul’s response: he calls the high priest a whitewashed wall. Now, this isn’t an insult without merit; a whitewashed wall is a concrete image of hypocrisy, giving the appearance of wholeness or cleanliness while actually trying to hide decay or dirt or damage. On the whole, Paul’s response is an extraordinarily sharp and shocking rebuke of the high priest.
Skipping the return rebuke leveled against Paul, in response he seems to make a complete reversal from his original response. This verse, however, presents a number of interpretive questions. To begin, if taken at face value, how in the world did Paul not know that the one who ordered him to be struck was not the high priest? Some say that Paul had been away from Jerusalem for so long he didn’t know Ananias; others say that Paul’s eyesight was bad, so that he didn’t know who ordered him struck. I’m inclined, though, to understand Paul’s words sarcastically.
Following Augustine and Calvin, Paul appears to be appreciating the irony that the true high priest has already been made known—the Lord Jesus Christ. Beyond that, Ananias’ actions clearly did not evidence his fitness for such an office in the first place. Finally, I wonder if there was irony in the fact that Paul himself had been appointed by the risen Lord as a ruler of sorts of his people—both an apostle and an overseer of the people of God. In a sham trial before hostile ears, Paul’s possibly sarcastic response only highlights the ridiculousness of the situation and Paul’s own innocence in the things that really matter.
And that’s an important observation because Paul’s strong language is not meaningless. Rather, it appears to be meant to shock those who ought to see the light and know the truth into taking a second look. Indeed, each of the examples mentioned above is directed at a person or group within or closely connected to the covenant community. That is to say, to those who are outsiders, Paul shows restraint. But within the covenant community, Paul is willing to use strong language to rebuke false teaching and bad practice.
And that’s consistent with the way that the Lord has engaged with his people throughout the Scriptures. For us, the point is that, while kindness and gentleness are fruit of the Spirit, that doesn’t mean that strong language rebuking false teaching or bad practice is excluded within the covenant community. Just make sure you endeavor to maintain a clear conscience before God lest you turn out to be a whitewashed wall yourself (cf. Matt 7:1-6)!
 Gordon D. Fee, Paul’s Letter to the Philippians, NICNT (Grand Rapids: Eerdmans, 1995), 319.