vv. 37-38 "For nothing will be impossible with God." Then Mary said, "Here am I, the servant of the Lord; let it be with me according to your word."
"For nothing will be impossible with God" is one of those dangerous little bits of Bible text. It sounds so good! We want to believe it just as it is said, a little "proof text." See, God can do anything.
But like all bits of the Bible it has a context which gives it meaning. One of those contexts for this saying is that first Elizabeth, earlier in the story, and then Mary have to say yes. God is "powerless" without their acquiescence. "For nothing will be impossible with God" depends on "let it be with me according to your word."
This tells us a couple things. The first is that God works through us. God rarely chooses to work around us. Such is God's commitment to us. The second is, the whole of our life is a constant saying "yes" or "no" to God. Sometimes when we say no it is because we do not yet have enough information. It is not yet clear to us what God is asking. That's OK as long as we are committed to the path of finding out. It would be nice if God sent us an angel to make it clear but that seems to be a very rare occurrence. For us, we just have to pay attention, look for signs, continue to feed ourselves with word and sacrament. God's desire will come to us. Our opportunity to say "let it be with me according to your word" will come.