What does Luke 1:49 mean?

When the angel Gabriel spoke to Mary (Luke 1:26–27), he made references to prophetic statements about the Messiah (Isaiah 9:6–7; 2 Samuel 7:12–16). His description of Jesus Christ (Luke 1:31–33) was not lost on Mary. Not only did she respond in humility and faith (Luke 1:28), but she also seems to have grasped the meaning of the angel's words. As she celebrates the news with her relative, Elizabeth (Luke 1:39–42), Mary demonstrates familiarity with Scripture. Her song of praise—sometimes called "The Magnificat"—includes many quotes and allusions to the Old Testament (Luke 1:46–55).

In the early part of her speech, Mary humbly worshipped God for His role as her Savior (Luke 1:46–47). Her words echoed some of Hannah's prayer to God, as she asked for a child in 1 Samuel 1:11 and celebrated an answer to prayer in 1 Samuel 2:1. Here, she again gives all credit for this miracle to God. She acknowledges God's power (Psalm 50:1; 62:7; 106:2) and holiness (Exodus 15:11; Leviticus 20:26; Psalm 99:5).

As this song continues, Mary will speak of God's use of the Messiah in the present tense. This is a common technique in psalms, in prophecy, and in praise of God. That which He promises to do will certainly be done—His decree is a guarantee. Many of God's messages declare his future acts as if they were already happening. In a similar way, Mary's celebration of Messiah's influence takes on the "as good as done" tone.