The Jews who opposed the Gospel (Acts 17:5) were probably the synagogue rulers, for Paul says that he preached the Gospel there through much contention (1Thessalonians 2:2). Since the opposition he endured came from the Jews (Acts 17:5) and not from gentiles, as was the case in Philippi (1Thessalonians 2:2), no doubt he was opposed from the very beginning when he shared Christ with the Thessalonians on three Sabbath days (Acts 17:3-4). Moreover, in view of the fact that the majority of the Jews did not receive the Gospel, it becomes even more persuasive that the synagogue rulers or leaders openly opposed Paul’s arguments (Acts 17:5) in the synagogue. Had the leaders received the Gospel, most Jews in Thessalonica would no doubt have gone along with their leaders.
Later, we are told that the folks in Beroea were more noble that those at Thessalonica in that they searched the Scriptures daily to see if Paul’s claims were true (Acts 17:10-11). This implies that those in Thessalonica never used the Scriptures to oppose Paul. They simply rejected the Gospel on its face, desiring rather the false doctrine about the Messiah, which they cherished over the truth.
Yet, their opposition to the Gospel didn’t end after Paul stopped coming to the synagogue, as was his manner, once the Gospel was rejected by the Jews. These folks worked successfully to get Paul expelled from the city by employing the services of certain malicious men in the marketplace (Acts 17:5). Luke uses this in contrast to the prominent women and devout God-fearing men in verse-4; this contrast would not have been lost to Theophilus, Luke’s intended reader (Acts 1:1), for the Annas family had always used such men to accomplish their devious ends.
Notice the form the opposition took in the marketplace and then before the city magistrates (Acts 17:6-8). First, Paul’s enemies claimed that they who have created disorder among the people all over the empire had at last come to their city (verse-6); and, secondly, they alleged that Paul was advocating measures contrary to the decrees of Caesar. Was this true, and, if not, how difficult would it be for Paul to prove it wasn’t so?
First of all, neither accusation was meant to be received as treasonous, because treason would have been against the law throughout the empire. Moreover, Paul could have easily shown he was not advocating the overthrow of the existing government. Normally the decrees of the emperor were not binding upon a free city in the empire, as was Thessalonica. However, recently, Caesar had expelled the Jews from Rome over a controversy involving one Chrestus  (Christ). If it was true that the expulsion was over the Gospel, as was the accusation in Thessalonica, then the synagogue leaders were referring to such incidents as this. Moreover, The Hellenist Jews had been expelled from Jerusalem (Acts 8:1), as had the Apostles under King Agrippa (Acts 12), and there were reports of seditious activity in Judea over a number of messianic claimants who promised to remove Roman rule over the Jews. Although this latter had nothing to do with Jesus, the accusation was meant to be inflammatory, and outsiders (i.e. gentiles in Thessalonica) probably would be unable to tell the difference between false messianic figures and Jesus.
Secondly, the claim that Paul preached Christ against the decrees of Caesar, is interesting, and probably the more difficult of the two to disprove. For example, Paul urged gentiles to receive Jesus and turn to the one true God (1Thessalonians 1:9), but the emperor also claimed to be god, and archeology shows the emperor-cult was active in these parts of Macedonia. The Gospel also advocated looking for the coming of the Christ (1Thessalonians 4:15), and Jesus was called the Son of God (1Thessalonians 1:10), just as was the emperor! Paul also preached that Jesus would usher in the “Day of the Lord” when he would judge both the Jews and the gentiles (1Thessalonians 5:3); and the Pax Romana of which Caesar boasted and brought to the empire would be in danger, if Paul’s Gospel was true.
Moreover, it has been suggested that the decrees of Caesar included such things as predictions of the replacement of existing rulers or of their death, which may have been inspired by the predictions of the Pharisees against Herod (Josephus: Antiquities 17.2.4); and Paul did claim a leader who opposed God would be destroyed at the Lord’s coming (2Thessalonians 2:8).
The city leaders in Thessalonica were more prudent than those at Philippi. They were not intimidated by the angry Jewish party, but neither could they simply ignore their claims. When Paul and Silas couldn’t be found, Jason, who opened his house to the missionary team, was taken before the city authorities instead. The city officials made Jason post what amounts to a peace bond for both his behavior (including that of other believers there) and probably to insure that he sent Paul and Silas away—i.e. they were expelled from the city and could not return while these same men held office.
 See Suetonius Claudius 25:4 and Acts 18:2.