Gabriel's response to Mary's question "How can this be seeing I don't know a man?" is being considered in two parts. The first of these was discussed in Luke's Testimony : Part 4. The second can be considered now:
"...wherefore also that holy thing which is begotten shall be called the Son of God."
There is nothing singular or exclusive about the word 'holy' in reference to Jesus. According to the Law of Moses, every first born male was declared 'holy' and had to be redeemed or brought back by his parents.
A comparison of texts will place the matter in its correct perspective:
"While they were there, the time came for the baby to be born, and she gave birth to her firstborn, a son." (NIV Luke 2:6-7)
"And YHWH spoke unto Moses saying, Sanctify unto me all the first-born, whatsover openeth the womb among the Children of Israel, both of man and beast: it is mine." (KJV Exodus 13:1-2)
"...the males that openeth the womb shall be YHWH's." (KJV Exodus 13:12)
"And when the days of their purification according to the Law of Moses were fulfilled, they brought him up to Jerusalem, to present him to the Lord (as it is written in the law of the Lord, Every male that openeth the womb shall be called 'holy' to the Lord)." KJV Luke 2:22-23
Coming up...A Child of Promise