“The Holy Spirit.” “The Holy Spirit” (capital “H,” capital “S”) is a name used for God when His power is in operation. In a very Hebraic way, this verse equates the Holy Spirit with “the power of the Most High.” The angel was speaking to Mary, a young Hebrew woman, in terms she could understand. It was common in the Hebrew language to say something and then repeat it in different words so the meaning would be clear. This occurs throughout the Hebrew Old Testament, and can especially be seen in books such as Proverbs in which something is stated and then restated using different words.
Since Mary told the angel she was not having sexual relations with a man, and knew she would then have to be impregnated by God, she would naturally understand “the Holy Spirit” to be the name of God which emphasized His invisible power in operation. God has many names in the Bible, and “the Holy Spirit” is one of them. It is easy to tell that in this case “the Holy Spirit” is a name of God because Jesus is called “the Son of God” and “the Son of the Father” (2 John 1:3), but he is never called “the Son of the Holy Spirit.” Mary understood that “the Holy Spirit” was another name for God, and thus she told her cousin Elizabeth that she rejoiced in God, and that “the Mighty One” (another name for God) had done great things for her (Luke 1:47-49).
[For more information on “the Holy Spirit,” see Appendix 11: “What is the Holy Spirit?”]