God’s Spirit has been available to man since the beginning. The Old Testament is full of examples of this (see below).
When Jesus went back to heaven, he sent The Comforter to be with us and a special Former Rain was given to the disciples who put away sin and were all of one mind at Pentecost to enable them to spread the gospel with power.
A special Latter Rain will be given as well near the very end to give the loud cry of Revelation 14 and finish the work so Jesus can return.
And Pharaoh said unto his servants, Can we find such a one as this is, a man in whom the Spirit of God is?
Genesis 41:38
And I have filled him with the spirit of God, in wisdom, and in understanding, and in knowledge, and in all manner of workmanship,
Exodus 31:3
And I will come down and talk with thee there: and I will take of the spirit which is upon thee, and will put it upon them; and they shall bear the burden of the people with thee, that thou bear it not thyself alone.
Numbers 11:17
And the LORD came down in a cloud, and spake unto him, and took of the spirit that was upon him, and gave it unto the seventy elders: and it came to pass, that, when the spirit rested upon them, they prophesied, and did not cease.
Numbers 11:25
And Balaam lifted up his eyes, and he saw Israel abiding in his tents according to their tribes; and the spirit of God came upon him.
Numbers 24:2
But the Spirit of the LORD came upon Gideon, and he blew a trumpet…
Judges 6:34
Then the Spirit of the LORD came upon Jephthah, and he passed over Gilead…
Judges 11:29
And the Spirit of God came upon Saul when he heard those tidings…
1 Samuel 11:6
Then Samuel took the horn of oil, and anointed him in the midst of his brethren: and the Spirit of the LORD came upon David from that day forward…
1 Samuel 16:13
And the Spirit of God came upon Azariah the son of Oded:
2 Chronicles 15:1
Yet thou in thy manifold mercies forsookest them not in the wilderness: the pillar of the cloud departed not from them by day, to lead them in the way; neither the pillar of fire by night, to shew them light, and the way wherein they should go.
Nehemiah 9:19,20
Thou gavest also thy good spirit to instruct them, and withheldest not thy manna from their mouth, and gavest them water for their thirst.
Cast me not away from thy presence; and take not thy holy spirit from me.
Psalm 51:11
And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
Ezekiel 36:27