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Cf. the identical phrase in verse {9} above, relating to the announcement of John's birth to Zachariah. In both these cases, the implication is that God can and does bring about events which may be utterly unexpected or even inconceivable before they materialize. In connection with the announcement of a son to Mary, the Qur'an states in {3 :47} that "when He wills a thing to be, He but says unto it, 'Be' - and it is": but since neither the Qur'an nor any authentic Tradition tells us anything about the chain of causes and effects (asbab) which God's decree "Be" was to bring into being, all speculation as to the "how" of this event must remain beyond the scope of a Qur'an-commentary. (But see also note [87] on {21: 91}.)
One of the several meanings of the term ayah is "a sign" or, as elaborately defined by Raghib, "a symbol" (cf. surah {17}, note [2]). However, the sense in which it is most frequently used in the Qur'an is "a [divine] message": hence, its metonymic application to Jesus may mean that he was destined to become a vehicle of God's message to man - i.e., a prophet - and, thus, a symbol of God's grace. - As regards the words "thou shalt have a son" interpolated by me between brackets, a statement to this effect is implied in the subsequent phrase beginning with "so that" (Zamakhshari and Razi).
The mission of Jesus is announced in two ways (1) he was to be a Sign to men; his wonderful birth and wonderful life were to turn an ungodly world back to Allah; and (2) his mission was similar to that of all prophets of Allah. But the point here is that the Israelites, to whom Jesus was sent, were a hardened race, for whom the message of Jesus was truly a gospel of Mercy.
For anything that Allah wishes to create, He says "Be", and it is (Cf. iii. 47). There is no interval between His decree and its accomplishment, except such as He imposes by His decree. Time may be only a projection of our own minds in this world of relativity.