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This parenthetic passage connects with verses {1-3} above, and relates to every prophet's "pledge" - i.e., sacred duty - to convey God's message to man, and thus to act as "a bearer of glad tidings and a warner". (For my rendering of idh, in this context, as "lo", see surah {2}, note [21].)
Cf. iii. 81. There is an implied covenant on all created things to follow Allah's Law, which is the law of their being; see v. 1. But there is a special implied convenant with all Prophets, strict and solemn, that they shall carry out their mission, proclaim Allah's Truth without fear or favour, and be ever ready in His service in all circumstances. That gives them their position and dignity as explained in the last verse, and their tremendous responsibility in respect of the people whom they come to instruct and lead to the right Path.