Question:
Answer:
The perfection of freedom is being impervious to—or being free from committing—sin. As the eternal Son of God who became man, Jesus Christ not only didn’t sin, he couldn’t have sinned. If he were vulnerable to sin, he would not have been God.
In turn, Mary was immaculately conceived, i.e., conceived without sin. But being immaculately conceived doesn’t mean she was not free to say no at the Annunciation. After all, she was merely human, despite her exalted status. Given her immaculate conception and her cooperation with God’s grace as she grew up, Mary was certainly inclined toward accepting God’s will for her life. But the Church doesn’t teach that she couldn’t have sinned, including at the Annunciation.
On the other hand, it’s a common theological opinion that Mary was confirmed in grace at the Annunciation, i.e., that she became impervious to sin the rest of her life. And yet she suffered greatly in her life for the sins of others, in concert with her divine Son.