24 And it was not in the Twelve Apostles only that the grace of the Holy Spirit wrought, but also in the first-born children of this once barren Church, I mean the seven Deacons; for these also were chosen, as it is written, being full of the Holy Ghost and of wisdom. Of whom Stephen, rightly so named, the first fruits of the Martyrs, a man full of faith and of the Holy Ghost, wrought great wonders and miracles among the people, and vanquished those who disputed with him; for they were not able to resist the wisdom and the Spirit by which he spoke. But when he was maliciously accused and brought to the judgment hall, he was radiant with angelic brightness; for all they who sat in the council, looking steadfastly on him, saw his face, as it had been the face of an Angel. And having by his wise defence confuted the Jews, those stiffnecked men, uncircumcised in heart and ears, ever resisting the Holy Ghost, he beheld the heavens opened, and saw the Son of Man standing on the right hand of God.
He saw Him, not by his own power, but, as the Divine Scripture says, being full of the Holy Ghost, he looked up steadfastly into heaven, and saw the glory of God, and Jesus standing on the right hand of God.
Source: Catechetical Lectures (New Advent)