Philip opened his mouth. — The phrase, wherever it occurs in the New Testament, implies something like a set discourse. (Comp. Acts 10:34; Acts 18:14; Matthew 5:2; Matthew 13:35; 2 Corinthians 6:11). It always means something more than the mere act of speaking.
And preached unto him Jesus. — The sequel shows that the teaching must have included, not only an interpretation of the prophecy as fulfilled in Christ, but instruction as to the outward condition of admission to the society of the disciples. The eunuch hears enough to make him eager for the baptism which was to bring with it so great a blessing.