Q. How could the same Spirit who descended at Pentecost have already been at work in the world since its creation?
A. God is present in His world in different ways and to different “degrees,” we might say. Scripture tells us that the Holy Spirit, who is God, was active in the creation of the world (see Genesis 1:2). He has remained present in the world as the One who renews His creation (Psalm 104:30) and inspires human creativity (Exodus 31:3; 35:31) and other gifts (Judges 3:10; 6:34).
On certain occasions throughout history, God the Holy Spirit has been present in what we might call a more “focused” way, when He has prompted people to speak on His behalf, most especially the prophets (see, for example, 2 Samuel 23:2; Isaiah 61:1-3). On these occasions, Scripture speaks of His “coming upon” someone or “filling” someone.
God the Holy Spirit was present in a unique way at the incarnation of God the Son. In a mystery we don’t fully understand, the Spirit came upon Mary so that she conceived Jesus “by the power of the Holy Spirit,” and in time gave birth to a Son who was both God and Man. The same Spirit filled St. Elizabeth and spoke through her and her husband, St. Zechariah (Luke 1:41–45; 67–79), and prompted St. Simeon to come to the temple to meet the Holy Family (2:27).
Years later, the Spirit “descended upon [Jesus] in bodily form like a dove” (Luke 3:21) at His baptism, so that Jesus was “filled with the Holy Spirit” (4:1). This doesn’t mean that before His baptism, Jesus did not have the Holy Spirit present with Him. It simply refers to a different kind of presence — among other things, a presence more clearly discernible to others.