Albert Barnes Commentary Luke 1:35

"And the angel answered and said unto her, The Holy Spirit shall come upon thee, and the power of the Most High shall overshadow thee: wherefore also the holy thing which is begotten shall be called the Son of God." — Luke 1:35 (ASV)

The Holy Ghost shall come upon you. Matthew 1:20.

The power of the Highest, and so on. This evidently means that the body of Jesus would be created by the direct power of God. It was not by ordinary generation; but, as the Messiah came to redeem sinners—to make atonement for others, and not for himself—it was necessary that his human nature should be pure, and free from the corruption of the fall. God therefore prepared him a body by direct creation that should be pure and holy. See Hebrews 10:5.

That holy thing, and so on. That holy progeny or child.

Shall be called the Son of God. This is spoken in reference to the human nature of Christ, and this passage proves, beyond controversy, that one reason why Jesus was called the Son of God was because he was begotten in a supernatural manner. He is also called the Son of God on account of his resurrection, Romans 1:4; Acts 13:33; Psalms 2:7.