What does John the Evangelist say about this divine promise?
Q411 · The Holy Spirit and Divine Salvation[792 ]
“But this He spoke concerning the Spirit, whom those believing in Him would receive; for the Holy Spirit was not yet given, because Jesus was not yet glorified” (John 7:39). God created the human being so that the Spirit of God could work within him, and he could abide in the Spirit. However, the human being chose to turn his back on God and reject Him; so the Spirit became more like a guest working in godly people and prophets, and even sometimes in non-believers, to attract them to faith. However, when the Lord was glorified on the cross and paid the price for the forgiveness of our sins so we may be glorified in Him, He gave the Spirit abundantly, as previously promised in Joel 2:28.
795 St. Ambrose, On the Holy Spirit 3.20.153–154 (NPNF II/10:156).
The Holy Spirit is the greatest gift, or rather, He is the giver of all the gifts we receive, through His descent upon the Church and His dwelling in the hearts of the faithful, widely bestowing gifts and talents as He sees fit, for the glory of God and for the growth of the Church and the salvation of souls.