Question:
Answer:
All Old Testament prophets had confidence in God’s word being fulfilled, even if they didn’t fully understand the prophetic words they spoke.
In the case of Jonah, for example, I think he had full confidence and comprehension that God would destroy Nineveh in forty days if the inhabitants didn’t repent (Jonah 3:1ff.). The king and people of Nineveh repented, and so God mercifully spared them. This upset Jonah, but he learned a good lesson about God’s mercy (Jonah 4:1ff.).
In the case of Isaiah and his prophecy that an almah—a young woman maiden or virgin—would have a child who would be called “Emmanuel,” which means “God is with us” (Isaiah 7:14), he wouldn’t have fully comprehended the importance of his words. As Matthew explains this prophecy, the passage from Isaiah 7 is fulfilled in the Blessed Virgin Mary’s conception of Jesus (Matt. 1:22-23).
Isaiah undoubtedly had confidence that a woman could conceive a child miraculously, i.e., by God’s grace, but he wouldn’t have understood that God is a Trinity, and that the second Person of the Trinity—the eternal Son of God—would be that special child. Such truth had not been revealed in Old Testament times, even though they were hinted at through various prophetic words.
The reality of Jesus’ divinity is arguably something that even his closest disciples—including his Blessed Mother—did not fully understand and believe until the Resurrection. To be clear, the Blessed Mother understood her Son was the Messiah and had full confidence God’s word would be fulfilled in him. But did she fully comprehend that her Son was the Incarnate Word prior to his resurrection? She arguably didn’t; but if anyone did, she would have, given her exalted status as his mother.
For more on the meaning of Isaiah 7:14 and its fulfillment in Mary’s conception of Jesus, please see Scott Hahn’s presentation on the subject.