What was God’s eternal Purpose that the Holy Spirit came to fulfill on Pentecost?

Why did God devise the plan of salvation through Jesus Christ?
Why was Jesus born in the earth as human?
Why did He die on the cross?
Why was He raised from the dead and ascended to Heaven?
Why did He sit at the right of His Father, on the throne of David?
What did God want to accomplish through all that plan?
How did HE make it happen?
What was the power that He sent to make it happen?
To whom did God send the power to fulfill His plan?
Who is NOW called to do the WORK that is needed to fulfill God’s plan?
What is that work that must be accomplished?

The Holy Spirit was poured out on Pentecost to give power to God’s people to overcome the forces of Darkness in order to do God’s will.

That was God’s ETERNAL PURPOSE from before the foundation of the world:

Ephesians 3:8 To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, 9 and to make all see what [is] the fellowship of the mystery, which from the beginning of the ages has been hidden in God who created all things through Jesus Christ; 10 to the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly [places,] 11 according to the eternal purpose which He accomplished in Christ Jesus our Lord,

Colossians 1:15 He is the image of the invisible God, the firstborn over all creation. 16 For by Him all things were created that are in heaven and that are on earth, visible and invisible, whether thrones or dominions or principalities or powers. All things were created through Him and for Him.
17 And He is before all things, and in Him all things consist.
18 And He is the head of the body, the church, who is the beginning, the firstborn from the dead, that in all things He may have the preeminence.
19 For it pleased [the Father that] in Him all the fullness should dwell, 20 and by Him to reconcile all things to Himself, by Him, whether things on earth or things in heaven, having made peace through the blood of His cross.