I couldn't have known this about this dear girl, God's lowly handmaid, apart from Scripture, that is, divine revelation: in this case, the highly revealing title by which God's angel addressed her.
If Mary was without sin as some believe, then Jesus could not be her Savior because she would have had nothing to be saved from. Throughout history, God has selected mortal, sinful men to carry out his plan, and Mary was greatly favored to be the mother of Jesus. But Mary was not a goddess, and needed the same grace for her salvation that we require.