I. The Holy Ghost's work did not begin at Pentecost, but was continued because the Holy Trinity is One in action and in will
Today's festival, dearly-beloved, which is held in reverence by the whole world, has been hallowed by that advent of the Holy Ghost, which on the fiftieth day after the Lord's Resurrection, descended on the Apostles and the multitude of believers, even as it was hoped. And there was this hope, because the Lord Jesus had promised that He should come, not then first to be the Indweller of the saints, but to kindle to a greater heat, and to fill with larger abundance the hearts that were dedicated to Him, increasing, not commencing His gifts, not fresh in operation because richer in bounty.
For the Majesty of the Holy Ghost is never separate from the Omnipotence of the Father and the Son, and whatever the Divine government accomplishes in the ordering of all things, proceeds from the Providence of the whole Trinity. Therein exists unity of mercy and loving-kindness, unity of judgment and justice: nor is there any division in action where there is no divergence of will. What, therefore, the Father enlightens, the Son enlightens, and the Holy Ghost enlightens: and while there is one Person of the Sent, another of the Sender, and another of the Promiser, both the Unity and the Trinity are at the same time revealed to us, so that the Essence which possesses equality and does not admit of solitariness is understood to belong to the same Substance but not the same Person.
Source: Sermons (New Advent)