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For a more detailed story of Abraham and the heavenly messengers, see verses {69-76} of surah {11} (Hud), which was revealed shortly before the present one. The connection between this story and the preceding stress on God's grace arises from Abraham's saying (in verse {56}), "And who - other than those who have utterly lost their way - could ever lose the hope of his Sustainer's grace?" Similarly, the subsequent references (in verses {58-84}) to the sinful communities that were destroyed because they refused to heed the warnings of their prophets are, obviously, meant to illustrate the reverse of God's grace, namely, His inevitable chastisement of deliberate, unrepented sinning (verse {50} above).
In illustration of the contrasts between Good and Evil, and the consequences that flow from them, we have now a reference to four incidents from the past, viz.: (1) an incident from the story of Abraham; (2) from that of Lot, nephew of Abraham, and the end of the Cities of the Plain, which he was sent to warn; (3) the People of the Wood; and (4) the People of the Rocky Tract (Hijr), after whom this Sura is called. As usual, the recital of Allah's abounding grace comes first.