By St. John Chrysostom
But why did the Holy Spirit come to them, not while Christ was present, nor even immediately after his departure, but, whereas Christ ascended on the fortieth day, the Spirit descended “when the day of Pentecost,” that is, the fiftieth, “was fully come?”(Acts 2:1)
And how was it, if the Spirit had not yet come, that He said, “Receive ye the Holy Spirit?” (John 20:22)
In order to render them capable and meet for the reception of Him. For if Daniel fainted at the sight of an Angel (Dan. 8:17), much more would these when about to receive so great a grace.
Either this then is to be said, or else that Christ spoke of what was to come, as if it came already; as when He said, “Tread ye upon serpents and scorpions, and over all the power of the devil.” (Luke 10:19)
But why had the Holy Spirit not yet come? It was fit that they should first be brought to have a longing desire for that event, and so receive the grace. For this reason Christ Himself departed, and then the Spirit descended. For had He Himself been there, they would not have expected the Spirit so earnestly as they did. On this account neither did He come immediately after Christ’s Ascension, but after eight or nine days. It is the same with us also; for our desires towards God are then most raised, when we stand in need. Accordingly, John chose that time to send his disciples to Christ when they were likely to feel their need of Jesus, during his own imprisonment.
Besides, it was fit that our nature should be seen in heaven, and that the reconciliation should be perfected, and then the Spirit should come, and the joy should be unalloyed. For, if the Spirit being already come, Christ had then departed, and the Spirit remained; the consolation would not have been so great as it was. For in fact they clung to Him, and could not bear to part with Him; wherefore also to comfort them He said, “It is expedient for you that I go away.” (John 16:7) On this account He also waits during those intermediate days, that they might first despond for awhile, and be made, as I said, to feel their need of Him, and then reap a full and unalloyed delight. But if the Spirit were inferior to the Son, the consolation would not have been adequate; and how could He have said, “It is expedient for you?” For this reason the greater matters of teaching were reserved for the Spirit, that the disciples might not imagine Him inferior.
From Homily 1 of Acts of the Apostles.