Remember the context of Is. 7.14. Yahweh tells Ahaz to ask for a sign as deep as the depths or as high as the heavens (v.11). He refuses, so the Lord declares that he himself would give a sign: the virgin conceiving and giving birth to Immanuel (v13-14). Here is Motyer’s comment on the sign of Immanuel.
“Even supposing that virgin is collective and that a rash of Immanuels appears in the land, such naming would be cynically dismissed in the place as the product of female hysteria and not seen as a heaven-sent sign.” Motyer notes from the context (v. 11) that the Immanuel-sign is as great as the depth of Sheol or high as heaven above. In other words, this sign is going to be very great. He answers the critics and says that what they posit (many Immanuels, or a “collective” virgin) is not a great sign at all.
He continues: “What a depressing anticlimax following the Lord’s expressed willingness to ‘move heaven and earth’ and Isaiah’s dramatic outburst about the Sovereign himself giving a sign! The passage requires something more and if we look to the wider context of this closely integrated section we find it. In 8.8 we read your land, O Immanuel. Nowhere else does the Old Testament exemplify ‘land’ with a possessive pronoun accompanied by the subject of the pronoun in the vocative.
Furthermore, the singular possessive is linked with ‘land’ as a political unit only in the case of kings (e.g. Dt. 2.31; 2 Sa. 24.13) , Israel personified or some other personification (e.g. Je. 2.15; Ho. 10.1), or of the Lord (e.g. 1 Ki. 8.36; Ezk. 36.5). Immanuel cannot be simply any child whatever. Also, how could an ‘ordinary’ child become the ground of security of the Lord’s people against the onset of the nations (8.10)? Finally, it is impossible to separate this Immanuel from the Davidic king whose birth delivers his people (9.4-7) and whose complex name includes the designation Mighty God (9.6). Heaven and earth will truly be moved. Isaiah foresaw the birth of the divine son of David and also laid the foundation for the understanding of the unique nature of his birth.”
Motyer also has a nice article in Tyndale Bulletin, 21 (1970) called “Content and Context in the Interpretation of Isaiah 7.14.” Context indeed helps in this highly debated passage (7.14)!
The above commentary is taken from The Prophecy of Isaiah (Downers Grove: InterVarsity Press, 1993), 85-6.