“The former treatise have I made, O Theophilus, of all that Jesus began both to do and to teach.” Why does he put him in mind of the Gospel? To intimate how strictly he may be depended upon. For at the outset of the former work he says, “It seemed good to me also, having had perfect understanding of all things from the very first, to write unto you in order.” Neither is he content with his own testimony, but refers the whole matter to the Apostles, saying, “Even as they delivered them unto us, which from the beginning were eyewitnesses and ministers of the word.” Having then accredited his account in the former instance, he has no need to put forth his credentials afresh for this treatise, seeing his disciple has been once for all satisfied, and by the mention of that former work he has reminded him of the strict reliance to be placed in him for the truth. For if a person has shown himself competent and trustworthy to write of things which he has heard, and moreover has obtained our confidence, much more will he have a right to our confidence when he has composed an account, not of things which he has received from others, but of things which he has seen and heard. For you received what relates to Christ; much more will you receive what concerns the Apostles.
What then, (it may be asked), is it a question only of history, with which the Holy Spirit has nothing to do? Not so. For, if “those delivered it unto us, who from the beginning were eyewitnesses and ministers of the word;” then, what he says, is theirs. And why did he not say, 'As they who were counted worthy of the Holy Spirit delivered them unto us;' but “Those who were eyewitnesses?” Because, in matter of belief, the very thing that gives one a right to be believed, is the having learned from eyewitnesses: whereas the other appears to foolish persons mere parade and pretension. And therefore John also speaks thus: “I saw, and bare record that this is the Son of God.” And Christ expresses Himself in the same way to Nicodemus, while he was dull of apprehension, “We speak that we do know, and testify that we have seen; and no one receives our witness.” Accordingly, He gave them leave to rest their testimony in many particulars on the fact of their having seen them, when He said, “And do ye bear witness concerning Me, because you have been with Me from the beginning.” The Apostles themselves also often speak in a similar manner; “We are witnesses, and the Holy Spirit which God has given to those that obey Him.”; and on a subsequent occasion, Peter, still giving assurance of the Resurrection, said, “Seeing we ate and drank with Him.” For they more readily received the testimony of persons who had been His companions, because the notion of the Spirit was as yet very much beyond them. Therefore John also at that time, in his Gospel, speaking of the blood and water, said, he himself saw it, making the fact of his having seen it equivalent, for them, to the highest testimony, although the witness of the Spirit is more certain than the evidence of sight, but not so with unbelievers. Now that Luke was a partaker of the Spirit, is abundantly clear, both from the miracles which even now take place; and from the fact that in those times even ordinary persons were gifted with the Holy Ghost; and again from the testimony of Paul, in these words, “Whose praise is in the Gospel”; and from the appointment to which he was chosen: for having said this, the Apostle adds, “But also appointed of the Churches to travel with us with this grace which is administered by us.”
Now mark how unassuming he is. He does not say, The former Gospel which I preached, but, “The former treatise have I made;” accounting the title of Gospel to be too great for him; although it is on the score of this that the Apostle dignifies him: “Whose praise,” he says, “is in the Gospel.” But he himself modestly says, “The former treatise have I made— O Theophilus, of all that Jesus began both to do and to teach:” not simply “of all,” but from the beginning to the end; “until the day,” he says, “in which He was taken up.” And yet John says, that it was not possible to write all: for “were they written, I suppose,” says he, “that even the world itself could not contain the books written.” How then does the Evangelist here say, “Of all?” He does not say “all,” but “of all,” as much as to say, “in a summary way, and in the gross;” and “of all that is mainly and pressingly important.” Then he tells us in what sense he says all, when he adds, “Which Jesus began both to do and to teach;” meaning His miracles and teaching; and not only so, but implying that His doing was also a teaching.
But now consider the benevolent and Apostolic feelings of the writer: that for the sake of a single individual he took such pains as to write for him an entire Gospel. “That you might have,” he says, “the certainty of those things, wherein you have been instructed.” In truth, he had heard Christ say, “It is not the will of My Father that one of these little ones should perish.” And why did he not make one book of it, to send to one man Theophilus, but has divided it into two subjects? For clearness, and to give the brother a pause for rest. Besides, the two treatises are distinct in their subject-matter.
But consider how Christ accredited his words by His deeds. Thus He says, “Learn of Me, for I am meek and lowly in heart.” He taught men to be poor, and exhibited this by His actions: “For the Son of Man,” He says, “has not where to lay His head.” Again, He charged men to love their enemies; and He taught the same lesson on the Cross, when He prayed for those who were crucifying Him. He said, “If any man will sue you at the law, and take away your coat, let him have your cloak also”: now He not only gave His garments, but even His blood. In this way He bade others teach. Wherefore Paul also said, “So as you have us for an example.” For nothing is more frigid than a teacher who shows his philosophy only in words: this is to act the part not of a teacher, but of a hypocrite. Therefore the Apostles first taught by their conduct, and then by their words; nay rather they had no need of words, when their deeds spoke so loud. Nor is it wrong to speak of Christ's Passion as action, for in suffering all He performed that great and wonderful act, by which He destroyed death, and effected all else that He did for us.
Source: Homilies on Acts (New Advent)