6 witnesses of Christ here.
And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures, Acts 17:2–3
Paul used the Old Testament to show that the promised Messiah had to suffer and then rise from the dead. Christ's death and Resurrection were the plan all along, not a failure.
How we know: the verse itself names Him
Christ — the Anointed One (Greek for Messiah)
Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ. Acts 17:3
Paul's central claim: the man Jesus is the Anointed One the prophets promised. Everything in the scriptures he opened pointed to Him.
How we know: the verse itself names Him
Whom Jason hath received: and these all do contrary to the decrees of Caesar, saying that there is another king, one Jesus. Acts 17:7
Enemies accuse the missionaries of treason for calling Jesus a king. Without meaning to, they state a truth: Jesus is the King whose kingdom outranks Caesar's.
How we know: the verse itself names Him · a Church study aid lists it
Then certain philosophers of the Epicureans, and of the Stoicks, encountered him. And some said, What will this babbler say? other some, He seemeth to be a setter forth of strange gods: because he preached unto them Jesus, and the resurrection. Acts 17:18
The Athenian philosophers sum up Paul's message in two words: Jesus and the Resurrection. The risen Christ was the heart of his preaching.
How we know: the verse itself names Him
Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead. Acts 17:31
'That man' is Jesus Christ, whom the Father has appointed to judge all people. Paul does not name Him to the Athenians here, but the whole speech builds to Him.
How we know: a footnote points to Him · a Church study aid lists it
Because he hath appointed a day, in the which he will judge the world in righteousness by that man whom he hath ordained; whereof he hath given assurance unto all men, in that he hath raised him from the dead. Acts 17:31–32
The Father raising Jesus from the dead is the proof that He is the appointed Judge. Some Athenians mocked the Resurrection, but it is the guarantee of everything Paul taught.
How we know: a footnote points to Him · a Church study aid lists it