John Chrysostom

Then Philip opened his mouth, and began at the same Scripture, and preached unto him Jesus. It is likely he had heard that He was crucified, and now he learns, that His life is taken away from the earth, and the rest that He did no sin, nor deceit in His mouth: that He prevailed to save others also: and who He is, Whose generation is unutterable.

Homily on Acts 19 ↗

Oecumenius

Therefore this is Jesus, and Philip proclaims this Scripture to the eunuch, that it is fulfilled in him; and the eunuch, having believed and immediately baptized, was sent to the regions of Ethiopia, proclaiming that which he himself had believed: that there is one God foretold by the prophets, and that this One's Son has already been made present according to humanity, and was led as a sheep to the slaughter, and the other things which the prophets say about him. …

Commentary on Acts ↗

Bede

But Philip opening his mouth, etc. Philip interpreted means 'the mouth of a lamp,' and it is a beautiful sense that the mouth of a lamp opened its mouth, while it brought forth the obscure things of prophecy into the light of knowledge. Although these circumlocutions according to history could signify that his speech at that time would be somewhat longer.

Commentary on Acts ↗