Acts 1:4

When did they hear? When he said: It is to your advantage that I go away. For if I do not go away, the Advocate will not come to you (Jn. 16:7); and again: I will ask the Father, and He will send you another …

The Passover affords a more than usually solemn day for baptism; when, withal, the Lord's passion, in which we are baptized, was completed. Nor will it be incongruous to interpret figuratively the fact that, when the Lord was about to celebrate the last Passover, He said to the disciples who were sent to make preparation, "You will meet a man bearing water." He points out the place for celebrating the Passover by the sign of water. After that, Pentecost is a most joyous space for conferring baptisms; wherein, too, the resurrection of the Lord was repeatedly proved among the disciples [Acts 1:3], and the hope of the advent of the Lord indirectly pointed to, in that, at that time, when He had been received back into the heavens [Acts 1:9], the angels told the apostles that "He would so come, as He had withal ascended into the heavens;" [Acts 1:11] at Pentecost, of course. But, moreover, when Jeremiah says, "And I will gather them together from the extremities of the land in the feast-day," he signifies the day of the Passover and of Pentecost, which is properly a "feast-day." However, every day is the Lord's; every hour, every time, is apt for baptism: if there is a difference in the solemnity, distinction there is none in the grace.

But why did the Holy Ghost 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?" And how was it, if the Spirit had not yet come, that He said, "Receive ye the Holy Ghost?" In order to render them capable and meet for the reception of Him. For if Daniel fainted at the sight of an Angel, 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 come already; as when He said, "Tread ye upon serpents and scorpions, and over all the power of the devil." But why had the Holy Ghost 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. […]

Consider also how necessary He made it for them to abide in Jerusalem, by promising that the Spirit should be granted them. For lest they should again flee away after His Ascension, by this expectation, as by a bond, He keeps them to that spot. But having said, "Wait for the promise of the Father, which ye have heard of Me," He then adds, "For John truly baptized with water, but ye shall be baptized with the Holy Ghost not many days hence." For now indeed He gives them to see the difference there was betwixt Him and John, plainly, and not as heretofore in obscure hints; for in fact He had spoken very obscurely, when He said, "Notwithstanding, he that is least in the kingdom of heaven is greater than he:" but now He says plainly, "John baptized with water, but ye shall be baptized with the Holy Ghost." And he no longer uses the testimony, but merely adverts to the person of John, reminding the disciples of what he had said, and shows them that they are now become greater than John; seeing they too are to baptize with the Spirit. Again, He did not say, I baptize you with the Holy Ghost, but, "Ye shall be baptized:" teaching us humility. For this was plain enough from the testimony of John, that it was Christ Himself Who should baptize: "He it is that shall baptize you with the Holy Ghost and with fire;" wherefore also He made mention of John. The Gospels, then, are a history of what Christ did and said; but the Acts, of what that "other Comforter" said and did. Not but that the Spirit did many things in the Gospels also; even as Christ here in the Acts still works in men as He did in the Gospels; only then the Spirit wrought through the Temple, now through the Apostles: then, He came into the Virgin's womb, and fashioned the Temple; now, into Apostolic souls: then in the likeness of a dove; now, in the likeness of fire. And wherefore? […]

Then, that the Apostles might not say, that they were always having it held out to them in promises (for indeed Christ had already discoursed much to them concerning the Spirit, that they should not imagine It to be an impersonal Energy or Operation), that they might not say this, then, He adds, "not many days hence." And He did not explain when, that they might always watch: but, that it would soon take place, He told them, that they might not faint; yet the exact time He refrained from adding, that they might always be vigilant. Nor does He assure them by this alone; I mean, by the shortness of the time, but withal by saying, "The promise which ye have heard of Me." For this is not, saith He, the only time I have told you, but already I have promised what I shall certainly perform. What wonder then that He does not signify the day of the final consummation, when this day which was so near He did not choose to reveal? And with good reason; to the end they may be ever wakeful, and in a state of expectation and earnest heed.