“IF I do not go away, the Comforter will not come to you; but if I do go, I will send him to you.”
He does not say this in the sense that he could not, while being on earth, give the Spirit to the disciples, since it is plainly there to read that when he appeared to them after his resurrection he breathed on them and said to them, “Receive the Holy Spirit”.
Rather it was because, while he was on earth and living among them bodily, they were not strong enough by themselves to raise their minds to the gifts of heavenly grace for which we should thirst.
When he had ascended into heaven, however, they transferred their every desire onto him just as much there, very much as they had heard in another place: “Where your treasure is, there your heart will be also”.
So being now made fit for the Spirit, they were not saddened by his death, but gladdened by the gift of his promise, with Luke bearing witness to it in saying:
“And so it was that as he blessed them, he departed from them, and was carried into the heavens. And they returned to Jerusalem worshipping, with great joy, and were always to found in the Temple, praising and blessing God.”
St Bede, Homily for the Third Sunday after Easter. (My translation.)
Priest: Let us lift up our hearts!
People: We lift them up unto the Lord.
We praise thee, we bless thee,
we give thanks unto thee, O Lord.
From the Anaphora in the Divine Liturgy.