As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and lighting on him. (Matthew 3:16)
So, this is a fairly well-known verse, right? I mean I don't really have it memorized but I recognize it. And if you go chronologically through the life of Jesus, the Spirit of God descended on him (as shown above), and then he did his days in the wilderness, gathered disciples, taught, was unjustly murdered, then rose from the dead, and then shortly thereafter the Spirit came to the disciples at Pentecost.
So the Spirit is a New Testament thing, right?
Not so. When I did a search for "Spirit of God" just now, 25 references came up, and only 11 of them are in the New Testament. Which means that the Spirit of God was active before Jesus became a human. Check it out:
...and he has filled him with the Spirit of God, with skill, ability and knowledge in all kinds of crafts (Exodus 35:31)
When Balaam looked out and saw Israel encamped tribe by tribe, the Spirit of God came upon him (Numbers 24:2)
When they arrived at Gibeah, a procession of prophets met him; the Spirit of God came upon him in power, and he joined in their prophesying.(Samuel 10:10)
The Spirit of God came upon Azariah son of Oded. (2 Chronicles 15:1)
Then the Spirit of God came upon Zechariah son of Jehoiada the priest. (2 Chronicles 24:20)
The Spirit lifted me up and brought me to the exiles in Babylonia in the vision given by the Spirit of God. Then the vision I had seen went up from me, (Ezekial 11:24)
Wow, that's pretty cool, right? I think it means that God has always been at work in our lives. And that He truly is the same yesterday, today and tomorrow. The Spirit of God may have come more widely upon believers after Jesus' death, but that doesn't mean He wasn't active before.
There's a lot more I need to study about the Spirit of God, but I think this is a good start. :)