We are his witnesses - The word αυτου, his, is omitted by AD, and several others of good note; the Syriac, all the Arabic, Ethiopic, and Vulgate. It does not seem to be necessary.

Of these things - Των ῥηματων τουτων, Of these transactions: i.e. of Christ's life and miracles, and of your murderous proceedings against him.

And so is also the Holy Ghost - In the gift of tongues lately communicated; and by his power and influence on our souls, by which we are enabled to give irresistible witness of our Lord's resurrection.

To them that obey him - We obey God, not you; and therefore God gives us this Spirit, which is in us a fountain of light, life, love, and power.

The Spirit of God is given to the obedient: in proportion as a man who has received the first influences of it (for without this he cannot move in the spiritual life) is obedient to those influences, in the same proportion the gifts and graces, the light, life, and power, of the Holy Spirit, are increased in his soul.

are.

Acts 5:29 Then Peter and the other apostles answered and said, We ought to obey God rather than men.

Acts 1:8 But you shall receive power, after that the Holy Ghost is come on you: and you shall be witnesses to me both in Jerusalem...

Acts 2:32 This Jesus has God raised up, whereof we all are witnesses.

Acts 10:39-41 And we are witnesses of all things which he did both in the land of the Jews, and in Jerusalem; whom they slew and hanged on a tree...

Acts 13:31 And he was seen many days of them which came up with him from Galilee to Jerusalem, who are his witnesses to the people.

Luke 24:47,48 And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem...

John 15:27 And you also shall bear witness, because you have been with me from the beginning.

2 Corinthians 13:1 This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established.

Hebrews 2:3 How shall we escape, if we neglect so great salvation; which at the first began to be spoken by the Lord...

and so.

John 15:26 But when the Comforter is come, whom I will send to you from the Father, even the Spirit of truth, which proceeds from the Father...

John 16:7-14 Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come to you...

Hebrews 2:4 God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost...

1 Peter 1:12 To whom it was revealed, that not to themselves, but to us they did minister the things...

whom.

Acts 2:4,38,39 And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance...

Acts 10:44 While Peter yet spoke these words, the Holy Ghost fell on all them which heard the word.

John 7:39 (But this spoke he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet given...

Gamaliel's Counsel
'Refrain from these men, and let them alone; for if this counsel or this work be of men, it will come to nought: 39. But if it be of God, ye cannot overthrow it; lest haply ye be found even to fight against God.'--ACTS v. 38, 39. The little that is known of Gamaliel seems to indicate just such a man as would be likely to have given the advice in the text. His was a character which, on its good side and by its admirers, would be described as prudent, wise, cautious and calm, tolerant, opposed to fanaticism …
Alexander Maclaren—Expositions of Holy Scripture: The Acts

Whether Whatever God Does Outside the Natural Order is Miraculous?
Objection 1: It would seem that not everything which God does outside the natural order of things, is miraculous. For the creation of the world, and of souls, and the justification of the unrighteous, are done by God outside the natural order; as not being accomplished by the action of any natural cause. Yet these things are not called miracles. Therefore not everything that God does outside the natural order is a miracle. Objection 2: Further, a miracle is "something difficult, which seldom occurs, …
Saint Thomas Aquinas—Summa Theologica

Whether Subjects are Bound to Obey their Superiors in all Things?
Objection 1: It seems that subjects are bound to obey their superiors in all things. For the Apostle says (Col. 3:20): "Children, obey your parents in all things," and farther on (Col. 3:22): "Servants, obey in all things your masters according to the flesh." Therefore in like manner other subjects are bound to obey their superiors in all things. Objection 2: Further, superiors stand between God and their subjects, according to Dt. 5:5, "I was the mediator and stood between the Lord and you at that …
Saint Thomas Aquinas—Summa Theologica