Bob Cornwall, “The Sign of Emmanuel – Lectionary Reflection for Advent 4A (Isaiah 7)” (12.12.22), bobcornwall.com:
Isaiah’s message has an original context. It speaks to the concern of that moment when Ahaz faces invasion by his neighbors, as well as perhaps a larger threat from the emerging empire to the north. So, while early Christians drew on this passage in creating a messianic theology featuring Jesus, Isaiah speaks to a real eighth-century political crisis. In other words, Isaiah doesn’t have in mind a sign that involves a child born in the first century. In other words, this is not a prophetic word about a child born in Bethlehem to a virgin.