“Until the day in which He was taken up, after that He, through the Holy Spirit, had given commandments unto the Apostles whom He had chosen. After He had given commandments through the Spirit”; i.e. they were spiritual words that He spoke unto them, nothing human; either this is the meaning, or, that it was by the Spirit that He gave them commandments. Do you observe in what low terms he still speaks of Christ, as in fact Christ had spoken of Himself? “But if I by the Spirit of God cast out devils”; for indeed the Holy Ghost wrought in that Temple. Well, what did He command? “Go ye therefore,” He says, “make disciples of all the nations, baptizing them into the Name of the Father, and of the Son, and of the Holy Ghost; teaching them to observe all things whatsoever I have commanded you.” (<!--<span class="stiki"></span>-->Ib. 28:19-20.) A high encomium this for the Apostles; to have such a charge entrusted to them, I mean, the salvation of the world! Words full of the Spirit! And this the writer hints at in the expression, “through the Holy Ghost” (and, “the words which I spoke unto you,” says the Lord, “are Spirit”); thus leading the hearer on to a desire of learning what the commands were, and establishing the authority of the Apostles, seeing it is the words of the Spirit they are about to speak, and the commandments of Christ. “After He had given commandments,” he says, “He was taken up.” He does not say, 'ascended;' he still speaks as concerning a man. It appears then that He also taught the Disciples after His resurrection, but of this space of time no one has related to us the whole in detail. St. John indeed, as also does the present writer, dwells at greater length on this subject than the others; but none has clearly related every thing (for they hastened to something else); however, we have learned these things through the Apostles, for what they heard, that did they tell. “To whom also He showed Himself alive.” Having first spoken of the Ascension, he adverts to the Resurrection; for since you have been told that “He was taken up,” therefore, lest you should suppose Him to have been taken up by others, he adds, “To whom He showed Himself alive.” For if He showed Himself in the greater, surely He did in the minor circumstance. Do you see, how casually and unperceived he drops by the way the seeds of these great doctrines?
“Being seen of them during forty days.” He was not always with them now, as He was before the Resurrection. For the writer does not say “forty days,” but, “during forty days.” He came, and again disappeared; by this leading them on to higher conceptions, and no longer permitting them to stand affected towards Him in the same way as before, but taking effectual measures to secure both these objects, that the fact of His Resurrection should be believed, and that He Himself should be ever after apprehended to be greater than man. At the same time, these were two opposite things; for in order to the belief in His Resurrection, much was to be done of a human character, and for the other object, just the reverse. Nevertheless, both results have been effected, each when the fitting time arrived.
But why did He appear not to all, but to the Apostles only? Because to the many it would have seemed a mere apparition, inasmuch as they understood not the secret of the mystery. For if the disciples themselves were at first incredulous and were troubled, and needed the evidence of actual touch with the hand, and of His eating with them, how would it have fared in all likelihood with the multitude? For this reason therefore by the miracles [wrought by the Apostles] He renders the evidence of His Resurrection unequivocal, so that not only the men of those times— this is what would come of the ocular proof— but also all men thereafter, should be certain of the fact, that He was risen. Upon this ground also we argue with unbelievers. For if He did not rise again, but remains dead, how did the Apostles perform miracles in His name? But they did not, say you, perform miracles? How then was our religion (ἔ θνος) instituted? For this certainly they will not controvert nor impugn what we see with our eyes: so that when they say that no miracles took place, they inflict a worse stab upon themselves. For this would be the greatest of miracles, that without any miracles, the whole world should have eagerly come to be taken in the nets of twelve poor and illiterate men. For not by wealth of money, not by wisdom of words, not by any thing else of this kind, did the fishermen prevail; so that objectors must even against their will acknowledge that there was in these men a Divine power, for no human strength could ever possibly effect such great results. For this He then remained forty days on earth, furnishing in this length of time the sure evidence of their seeing Him in His own proper Person, that they might not suppose that what they saw was a phantom. And not content with this, He added also the evidence of eating with them at their board: as to signify this, the writer adds, “And being at table with them, He commanded.” And this circumstance the Apostles themselves always put forth as an fallible token of the Resurrection; as where they say, “Who ate and drank with Him.”
And what did He, when appearing unto them those forty days? Why, He conversed with them, says the writer, “concerning the kingdom of God.” For, since the disciples both had been distressed and troubled at the things which already had taken place, and were about to go forth to encounter great difficulties, He recovered them by His discourses concerning the future. “He commanded them that they should not depart from Jerusalem, but wait for the promise of the Father.” First, He led them out to Galilee, afraid and trembling, in order that they might listen to His words in security. Afterwards, when they had heard, and had passed forty days with Him, “He commanded them that they should not depart from Jerusalem.” Wherefore? Just as when soldiers are to charge a multitude, no one thinks of letting them issue forth until they have armed themselves, or as horses are not suffered to start from the barriers until they have got their charioteer; so Christ did not suffer these to appear in the field before the descent of the Spirit, that they might not be in a condition to be easily defeated and taken captive by the many. Nor was this the only reason, but also there were many in Jerusalem who should believe. And then again that it might not be said, that leaving their own acquaintance, they had gone to make a parade among strangers, therefore among those very men who had put Christ to death do they exhibit the proofs of His Resurrection, among those who had crucified and buried Him, in the very town in which the iniquitous deed had been perpetrated; thereby stopping the mouths of all foreign objectors. For when those even who had crucified Him appear as believers, clearly this proved both the fact of the crucifixion and the iniquity of the deed, and afforded a mighty evidence of the Resurrection. Furthermore, lest the Apostles should say, How shall it be possible for us to live among wicked and bloody men, they so many in number, we so few and contemptible, observe how He does away their fear and distress, by these words, “But wait for the promise of the Father, which you have heard of Me.” You will say, When had they heard this? When He said, “It is expedient for you that I go away; for if I go not away, the Comforter will not come unto you.” And again, “I will pray the Father, and He shall send you another Comforter, that He may abide with you.”
Source: Homilies on Acts (New Advent)