I had almost passed this thread by without any comment. But at the last moment gave in to the urge to chime in.
I am fully aboard Calvin's basic sentiment, which makes me along with him, under the estimate of the arch-theonomist RJR, guilty of "heretical nonsense." A label I consider a badge of honor. The penal sanctions of the Law of Moses are not of themselves morally binding, but judicial; and subject to considerations of wisdom discovering any general equity they may contain.
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Bringing up David's case together with the woman's (she caught in adultery) seems more than fair. David was the highest judicial authority in the land, and not above God's Law and jurisdiction; but at the same time not subject to a human court. There was none, but perhaps the high priest could have rebuked him as a peer; certainly Nathan did as the divine spokesman.
In the last analysis, God's verdict upon David is clear. The king pronounced his own death sentence on himself (unwittingly, in advance) in 2Sam.13:5, and admitted his guilt in v13. The same v has the prophet announcing that the king should not be put to death, because the LORD the King pardoned him.
God was free to grant this pardon. It was his Law given to men to administer, and he was not bound to abide by it. That David's sin would be paid for by the future Mediator and Sacrifice is implicit, inasmuch as "he will by no means clear the guilty," that is leave any sin unpunished.
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The case of the woman taken in adultery has other parallels in the ministry of Jesus, where his enemies sought to put him in a bind. Could they fashion a trap for him; whereby anything he said in response would trouble him with the people, with the religious authorities, or with the politicals?
Jesus avoided every pitfall in this crafty bind. Some of his words and acts are open to several lines of analysis. The missing man-culprit reveals a corrupt selective prosecution motive, and the possibility that the woman was made a patsy. His toying in the dust of the temple (Jn.8:2, 6, 8) calls to mind the ritual of trial for adultery, see Num.5:17.
But above all, while his rhetorical device of calling on the innocent ones to cast the first stone drives the accusers away; the actual claim of Jesus in this passage is one of divinity. He sets aside the woman's due penalty, not because she is innocent (she is not, and doesn't protest to be); and not so much on account of a lack of two or three witnesses remaining. He sets it aside because as God and as Messiah, he has the power to suspend her sentence: exactly the same as the LORD's power to revoke David's death sentence.
Ordinary justice could have, would have, proceeded against this woman; perhaps even caught up with her lover, if only to satisfy an equitable result, had Jesus or someone else brought that matter to the fore. No argument could be given that one guilty party in an adulterous situation was unpunishable, if the counterpart in the sin was escaped. That's like saying capturing one member of a gang of thieves will not yield a trial unless all his fellows are also captured.
King David showed mercy to his own son, guilty of an equally egregious sexual offense as this woman's (some will argue: more offensive). We could spend a long time debating the folly of his action (or inaction); wondering if any similar crime would have been pardoned in Israel's justice economy, perpetrated by someone other than a prince; wondering if any such pardon could ever be judged "wisdom" on the part of a human judge; or whether the king of Israel was compelled to condemn (and might never pardon) such crimes as the Law described as Most Heinous.
But the simple fact is that the Executive action of the king was done as God's anointed, and as a rule was not reviewable in this life. It was as the very judgment of God pronounced. I can only think of one case when the king's "justice" was invalidated by a mass repudiation of it by the people, 1Sam.14:44-45. The holy body experienced a kind of gag-reflex, when the head tried to force this injustice down its throat.
It wasn't too long ago, we were discussing on the PB the question of whether, or in what manner or condition or prudence, Jesus was under or was over the Law of Moses. Jesus is always consistent with his Father's will, always obedient to whatever moral righteousness a man--who is also king--ought to do. But Moses was a servant in Jesus' house, never the other way around. The Lord picked particular moments during his ministry to make it clear to various parties which of the two was in charge.
In the case of the woman taken in adultery, Jesus does nothing to subvert the Law, or even give that appearance. But the pardon he gives should not be viewed in any way other/less than his Kingly prerogative.