And Paul went in, as was his custom, and on three Sabbath days he reasoned with them from the Scriptures, explaining and proving that it was necessary for the Christ to suffer and to rise from the dead, and saying, “This Jesus, Whom I proclaim to you, is the Christ.” – Acts 17:2-3 (ESV)
The setting here is Thessalonica, and the place into which the Apostle Paul went for the reasoning, explaining, and proving was a “synagogue of the Jews” (17:1b). This was the heart of Paul’s mission. This is what concerned Paul, himself a Jew, who had been, along with nearly every other Jew in his day, awaiting the Messiah---awaiting God’s entering into history to vindicate and redeem His covenant people. Having come to terms with the fact that Jesus was the long-awaited Messiah, what else could he do, especially considering that, owing to the experience of the Damascus road, he had been specifically appointed to this task by Jesus Himself.
So Paul is in the synagogue, reasoning with the Jews about Jesus. Most would have been quite skeptical of his claims. It was not that Jesus was not liked and appreciated, the problem was that the story that was being told about Jesus did not fit with Jewish expectations concerning the expected Messiah (the Christ). Among many other things, the Messiah was to be a king in the mold of David and of Solomon. The Messiah was to have overthrown the oppressors of their nation, returning control of the land to the people of Israel. Contrary to all Messianic expectations, this Jesus fellow had been crucified, just like so many potential and failed messiahs that had come before Him.
The fact that Jesus had been crucified, regardless of all that He had said and done, was all the proof that the Jews of the day needed to establish the fact that, no, he was not the Messiah. He had not overthrown the enemies of Israel. He had been overthrown and cut down by those enemies in the way that they dealt with all challengers to the power of Rome and Caesar, by death at their hands on the cross. He had engaged the hope of the people but been cut down. Big deal. Nothing to see here. Time to move on. Owing to these things, Jesus could not possibly have been the Messiah, so what more did they need to know?
This is the mindset that was faced by Paul. It was not that the Jews were opposed to the message of Jesus. Some were, but this would not have been true for the vast majority. As far as they would have been concerned, this Jesus, while His ministry had been noble, had let them down like all the others, by not being a new Moses, leading a new exodus of sorts, resulting in the re-establishment of Israel as sovereign, autonomous, and independent in their land of promise.
Based on all that they had been taught, had seen, or was relatively fresh in the collective memory of the Jewish people, Jesus did not meet the requirements of Messiah-ship, especially since He had been killed at the hands of the Romans, which would have been the most important point in the argument against the fact that the disciples of Jesus, Paul himself, and the other followers of “the Way” were proposing. This is why Paul’s main task was to reason with them, from the Scriptures, and in doing so, show them, explain to them, prove to them, that it must be understood that Jesus’ death on the cross did not preclude Him from being the Messiah, but that His suffering and death (and therefore the Messiah’s death) was necessary according to the Scriptures.