11 Now why I have wished to bring this subject before you, give heed, Dearly Beloved. It was meet that God should first show that He works by the ministry of men; but afterwards by Himself, lest men should think, as Simon thought, that it was man's gift, and not God's. Though the disciples themselves knew this well already. For there were one hundred and twenty men collected together, when without the imposition of any hand the Holy Ghost came upon them. For who had laid hands on them at that time?
And yet He came, and filled them first. After that offense of Simon, what did God do? See Him teaching, not by words but by things. That same Philip, who had baptized the men, and the Holy Ghost had not come upon them, unless the Apostles had met together and laid their hands upon them, baptized the officer, that is, the eunuch of queen Candace, who had worshipped in Jerusalem, and returning thence was reading in his chariot Isaiah the Prophet, and understood it not. Philip being admonished went up to his chariot, explained the Scripture, unfolded the faith, preached Christ. The eunuch believed on Christ, and said when they came unto a certain water, “See water, who does hinder me to be baptized?
Philip said to him, Do you believe in Jesus Christ? He answered, I believe that Jesus Christ is the Son of God. Immediately he went down with him into the water.” When the mystery and sacrament of Baptism had been accomplished, that the gift of the Holy Ghost might not be thought to be of men, there was no waiting, as in the other case, for the Apostles to come, but the Holy Ghost came immediately. Thus was Simon's thought destroyed, lest in such a thought he might have followers.
Source: Sermons on Selected Lessons of the New Testament (New Advent)