Exactly like what we see in Philip with the eunuch, as he was reading the testimony of the prophets of Christ (I edited this, just added more)
"They are they which testify of me"
Isaiah 53:7 He was oppressed, and he was afflicted, yet he opened not his mouth:
he is brought as a lamb to the slaughter,
and as a sheep before her shearers is dumb, so he openeth not his mouth.
Shown as the scripture the eunuch was reading in Acts 8:32 the one question unknown to him was who (specifically) was the prophet speaking of as shown in Acts 8:34
Acts 8:34 And the eunuch answered Philip, and said, I pray thee,
"of whom" speaketh the prophet (even Isaiah) this?
of himself,
or of "some other" man?
He just didn't know who the prophet Isaiah was speaking of
But as Jesus had said (of the law and the prophets)
John 5:39 Search the scriptures; for in them ye think ye have eternal life:
and they are they which testify of me. (1John 1:2)
The Eunuch wasn't alone in this (See Acts 13:27)
But now (having their eyes opened) we see the same being expounded through Philip
Acts 8:35 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.
So it shows they preached Christ out of the prophets of old (which testified not only of him, but of what had just come to pass in their own day concerning his suffering and then entering into glory)
And we know this because they ask him earlier (not knowing it was him) this...
Luke 24:18... Art thou only a stranger in Jerusalem,
and hast not known the things which are come to pass there in these days?
Being somewhat surprised because of their hopes, and they went on to recount what had occurred in their days (with Jesus Christ)
And then Jesus reproves them in those very things
Luke 24:25 Then he said unto them, O fools,
and slow of heart to believe all that the prophets have spoken:
Luke 24:26 Ought not Christ to have suffered these things, and to enter into his glory?
Luke 24:27 And beginning at Moses and all the prophets,
he expounded unto them in all the scriptures the things concerning himself.
As is also confirmed by them in Acts here
Acts 3:18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled.
Even here this way
Acts 3:24 Yea, and all the prophets from Samuel and those that follow after,
as many as have spoken, have likewise foretold of these days.
Then Jesus commissions them (as God has given Jesus as commander to them as shown in Isaiah 55:4) the one Moses wrote of (as Jesus said in John 5:46) shown here (in Duet 18:18-19) And confirmed in Acts 3:22 & Acts 7:37, which helps us to understand why Jesus said this here (In John 12:48- 49) and what is said here in Acts 3:24 Lastly, as we can springboard off of all the above seeing in Jesus praying to the Father in confirmation of the same, that "I have given unto them the words which thou gavest me" (even as the doctrine he preached was not his, but his that sent Him). And as the Father sent him so did he (their commander) as God the Father made him so sends them.
Mark 16:16 And he said unto them,
Go ye into all the world, and preach the gospel to every creature.
Faith cometh by hearing and hearing by the word of God, as the voices of the prophets testify of Jesus Christ through whom they did preach him
And he has given commandment to these saying,
Mark 16:17 He that believeth and is baptized shall be saved; but he that believeth not shall be damned.
And then we see...
Mark 16:20 And they went forth, and preached every where...
Same with how Philip was led to preach the same to eunuch
Acts 8:29 Then the Spirit said unto Philip, (being led by the Spirit) said
Go near, and join thyself to this chariot. (who was reading the book of Isaiah)
Acts 8:30 And Philip ran thither to him,
and heard him read the prophet Esaias, and said, Understandest thou what thou readest?
And Philip preached Jesus out of Isaiah
Acts 8:35 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.
And even what Jesus said, even as he understood it, concerning being baptized because we see that he is made aware of this
Acts 8:36 And as they went on their way, they came unto a certain water: and the eunuch said, See, here is water; what doth hinder me to be baptized?
Acts 8:37 And Philip said, If thou believest with all thine heart, thou mayest (be baptized). And he answered and said, I believe that Jesus Christ is the Son of God.
Acts 8:38 And he commanded the chariot to stand still: and they went down both into the water, both Philip and the eunuch; and he baptized him.
Acts 8:39 And when they were come up out of the water, the Spirit of the Lord caught away Philip, that the eunuch saw him no more: and he went on his way rejoicing.
At least we know where the Eunuch started reading, which was the book of Isaiah (these having the testimony of Christ in themselves). And as faith cometh by hearing and hearing by the word of God (Jesus Christ is preached out of the same) as we see Philip beginning at the scripture written of him in Isaiah. Preaching Jesus Christ from them (so that the eunuch could believe). Philip having mentioned being baptized because the eunuch specifically brought this up to him (in his desire to be so) when he saw water and so he was. He both believed in Jesus Christ (through the testimony of the former prophets) God having fulfilled the same in those days even by Jesus Christ (of whom) they were all living witnesses to and he was baptized and went on his way rejoicing.
And just as we have Phillip expounding Jesus Christ out of the prophets
We see also in Apollos it shows
Acts 18:28 For he mightily convinced the Jews, and that publickly,
shewing by the scriptures that Jesus was Christ.
We see Paul expounding and testifying of the kingdom of God
Acts 28:23... "persuading them" concerning Jesus, "both out of the law of Moses, and out of the prophets", from morning till evening.
Even as our Lord said,
John 5:39 ...The scriptures... and they are they which testify of me.
The one leading you to Christ since he can be expounded out of them.