Commentators unanimously highlight that Mary is called "blessed" because she believed what God promised her. This isn't just about Mary; it's a universal principle for all believers. Scholars like John Calvin explain that while God's promises are true regardless, faith is the means by which we personally receive and experience their fulfillment. As multiple sources note, Mary's faith stands in stark contrast to the doubt of Zechariah, demonstrating that belief opens the door to God's blessings.