19 And the Lord Jesus, after he had spoken to them, was taken up into heaven, and sitteth on the right hand of God.
Gregory: Stephen, when toiling in the contest, saw Him standing [Acts 7:55], whom he had for his helper; but Mark describes Him as sitting after His assumption into heaven, because after the glory of His assumption, He will in the end be seen as a judge.
20 But they going forth preached every where: the Lord working withal, and confirming the word with signs that followed.
Augustine: How was this preaching fulfilled by the Apostles, since there are many nations in which it has just begun, and others in which it has not yet begun to be fulfilled? Truly then this precept was not so laid upon the Apostles by our Lord, as though they alone to whom He then spoke were to fulfil so great a charge; in the same way as He says, “Behold, I am with you always, even unto the end of the world” [Matt 28:20], apparently to them alone; but who does not understand that the promise is made to the Catholic Church, which though some are dying, others are born, shall be here unto the end of the world?
Theophylact: But we must also know from this that words are confirmed by deeds as then, in the Apostles, works confirmed their words, for signs followed. Grant then, O Christ, that the good words which we speak may be confirmed by works and deeds.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. II (London: Rivington, 1842).