It was the Fulfillment of the Divine Promise.
First, of the Father himself. During the Old Testament dispensation, he declared, again and again, that he would pour out the Spirit upon his people (see Prov. 1:23; Isa. 32:15; Joel 2:28, etc.); and now these gracious declarations were accomplished.
Second, of John the Baptist. When he was stirring the hearts of the multitudes by his call to repentance and his demand of baptism, many thought he must be the long expected Messiah, but he declared unto them, "I indeed baptize you with water, but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Spirit and with fire" (Luke 3:15, 16). Accordingly he did so on the day of Pentecost, as Acts 2:32, 33 plainly shows.
Third, of Christ. Seven times over the Lord Jesus avowed that he would give or send the Holy Spirit: Luke 24:49; John 7:37-39; 14:16-19; 14:26; 15:26; 16:7; Acts 1:5, 8. From these we may particularly notice, "When the Comforter is come, whom I will send unto you from the Father... he shall testify of me" (John 15:26): "It is expedient for you that I go away; for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you" (John 16:7). That which took place in John 20:22 and in Acts 2 was the fulfillment of those promises. In them we behold the faith of the Mediator: he had appropriated the promise which the Father had given him, "Therefore being by the right hand of God exalted, and having received of the Father the promise of the Holy Spirit, he hath shed forth this, which ye now see and hear" (Acts 2:33)—it was by faith’s anticipation the Lord spoke as he did in the above passage.
The Holy Spirit was God’s ascension gift to Christ, that he might be bestowed by Christ, as his ascension gift to the church. Hence Christ had said, "Behold, I send the promise of my Father upon you." This was the promised gift of the Father to the Son, and the Saviour" s promised gift to his believing people. How easy now to reconcile the apparent contradiction of Christ’s earlier and later words: "I will pray the Father and he shall give you another Comforter"; and then, afterward, "If I depart, I will send him unto you." The Spirit was the Father’s answer to the prayer of the Son; and so the gift was transferred by him to the mystical body of which he is the head (A. T. Pierson in The Acts of the Holy Spirit).