Philip, opening his mouth. To open the mouth is taken in Scripture for, to begin a long speech concerning some grave and weighty matter. Therefore Luke's meaning is, that Philip began to intreat [discourse] of Christ, as it were, with full mouth. He saith that he began with this prophecy, because there is no one which depainteth out Christ more lively; [1] and it was then brought [2] to his hand. Therefore, after that Philip had showed, by the prophet's words, after what sort Christ should come, and what was to be hoped for at his hands, he conferred the thing itself afterward, that the eunuch might know that that Christ which was promised was already revealed and given, and that he might understand his power. Where we translate it, that he preached Christ, Luke saith that he preached the gospel. The sense is, that he taught that of Christ which he uttered in his gospel himself, and commanded to be taught; whereby we gather, that when Christ is known, we have the sum of the gospel.

Footnotes

1 - "Clarius," more clearly.

2 - "Commode," conveniently, omitted.

Opened his mouth - See Matthew 5:2.
At the same scripture - Taking this as a "text" to be illustrated.
Preached unto him Jesus - Showed him that Jesus of Nazareth exactly corresponded to the description of the prophet, and that therefore he referred to the Messiah, and that the Messiah was Jesus of Nazareth. How far Philip detailed the circumstances of the life and death of Christ is unknown. What follows shows also that he stated the design of baptism, and the duty of being baptized.

Began at the same scripture - He did not confine himself to this one scripture, but made this his text, and showed, from the general tenor of the sacred writings, that Jesus was the Christ, or Messiah; and that in his person, birth, life, doctrine, miracles, passion, death, and resurrection, the Scriptures of the Old Testament were fulfilled. This preaching had the desired effect, for the eunuch was convinced of the truth of Philip's doctrine, and desired to be baptized in the name of Jesus.

Then Philip opened his mouth,.... With freedom and boldness, and spoke clearly and distinctly, and expounded the passage to him. This is a phrase frequently used in Jewish writings, especially in the book of Zohar (e), when they give an account of this or the other Rabbi interpreting any place of Scripture; as for instance, R. Eleazar "opened", and said, "my sabbaths you shall keep", &c. Again, R. Aba "opened", and said, "and Manoah said unto his wife", &c. Once more, "R. Simeon opened", and said, "the heavens declare the glory of God", &c. and
began at the same Scripture: "the fifty third of Isaiah": and preached unto him Jesus of Nazareth; how that he was the true Messiah, and the person there designed, and that very rightly; for the Jews themselves, the ancient ones, have interpreted several passages in that chapter of the Messiah, as has been before observed; he showed him how that he was born of a virgin, made under the law, and yielded perfect obedience to it in his life; and that he suffered and died for the sins of his people, and made satisfaction for them, and brought in an everlasting righteousness to justify them before God, and had obtained eternal redemption for them; that he was risen from the dead, was ascended into heaven, and now sat at the right hand of God, and ever lived to make intercession for transgressors; and would come a second time to judge both quick and dead; and in the mean while had left rules for the gathering, and forming, and governing his churches; and had appointed persons to preach his Gospel, and to administer the ordinances of baptism, and the supper of the Lord, which were to continue till his second coming.
(e) In Genesis. fol. 5. 3. 4. & 6. 1. 3. & 7. 1. 2. 3. & 8. 1. 2. 3. & passim.

Then Philip opened his mouth--(See on Matthew 5:2).
began at the same scripture--founding on it as his text.
preached unto him Jesus--showing Him to be the glorious Burden of this wonderful prediction, and interpreting it in the light of the facts of His history.