In the Old Testament, the Spirit of holiness is mentioned but not the Holy Spirit. The Holy Spirit is applied first to John the Baptist. This title is used in the conception of John the Baptist to introduce God's becoming a man in His incarnation (Luke 1:13-17). Luke 1:15 says concerning John the Baptist, "He will be filled with the Holy Spirit, even from his mother's womb." The preparing of the way for the Savior's coming required that His forerunner, John the Baptist, be filled with the Holy Spirit even from his mother's womb, so that he could separate the people unto God from all things other than God, making them holy unto Him for His purpose.
The beginning of the New Testament gives us a record of two conceptions. One was the conception of John the Baptist and the other was the conception of the Lord Jesus in God's incarnation to be a man in the flesh (Luke 1:30-36; Matt. 1:18-20). With these two conceptions, the New Testament uses the particular title the Holy Spirit. The Holy Spirit is used in the New Testament due to the change of the age. In order for God to become a man so that man could become God, there was the need of the Holy Spirit. The Old Testament was an age of figures and types, but the New Testament is the time of fulfillment, the age of reality, in which God became a man by being begotten of the Holy Spirit into humanity (Matt. 1:18, 20).
We need to see that the conception of John the Baptist was strikingly different in essence from that of Jesus the Savior. With John's conception, the essence of the Holy Spirit was not involved but the power. The conception of John was by the power of the Holy Spirit through man's instrument. But with the conception of Jesus, the very essence of the Holy Spirit Himself was involved. Thus, Matthew 1:20 says concerning the begetting of Jesus in Mary, "That which has been begotten in her is of the Holy Spirit." The conception of John the Baptist was God's miracle, accomplished with the human essence, merely by the divine power without the involvement of the divine essence. This resulted in the bringing forth of a mere man who was filled with the Spirit of God but who lacked the nature of God. The conception of the Savior was God's incarnation (John 1:14), constituted not only by the divine power but also of the divine essence added to the human essence, thus producing the God-man of two natures—divinity and humanity. These two conceptions are related to the beginning of God making Himself man and of God making man Him that He might become man and man might become Him, that the two could be one entity.