Luke 1:35
And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God. And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.
The angel's announcement to Mary told of the conception and birth of Jesus, and she treasured that word as evidence that He was the Messiah, the Sent of God. The coming of the Holy Spirit and the overshadowing of the power of the Highest meant the Holy Spirit coming upon Mary through God's power to bring about the conception. That conception and birth were a miracle, and the child to be born was holy and was to be called the Son of God.
He was already the only-begotten Son of God from all eternity, sustaining a relation to Jehovah before the foundations of the world were laid. By being born as a human child into the human race, while being the son of a human being, He became the Son of God in a new sense and stands in this world as the Son of God allied by birth to humanity. His divine origin did not make His humanity unreal, for He took real human nature with humanity joined to divinity, and the incarnation remains a mystery not fully explained to human beings. There was in Him no evil inclination, propensity, or taint of corruption.
In that humanity He assumed the liabilities of human nature to be proved and tried, was tempted as humans are under Satan's temptations, yet never yielded but lived a sinless life and so condemned sin in human flesh. He took responsibility for the human race and became liable for the sins of those who believe. By obeying every detail of God's law He showed that those who accept Him as their sin-offering and personal Saviour may receive His righteousness, share in the divine nature, and through His imputed righteousness receive power to overcome, obey God's commandments, and live in loyalty to their Creator. The language of overshadowing also applies as a prayer that God's Spirit and grace will be with gathered believers and bring them into living connection with God.