Who were Paul's opponents in early Christianity, and does anyone wonder why he had so many opponents? Does anyone else find it strange that an apostle would have people actually resist him? Wouldn't his authority be nearly absolute?
In Acts 6:13, Stephen was falsely accused of teaching against the Law, and in Acts 21:20-24, Paul took steps at the direction of James to disprove false rumors that he was teaching against the Law and to show that he continued to live in obedience to it, so if no one in leadership was teaching Jews against the Law, then all Christians were Torah observant Jews for roughly the first 7-15 years after Christ's ascension up until the inclusion of the Gentiles in Acts 10. While they did not teach against God's Law, they did teach against Jewish oral laws or traditions that were being taught as God's Law, and herein lies the confusion and the power struggle.
In Matthew 15:2-3, Jesus was asked why his disciples broke the traditions of the elders and he responded by asking them why they broke the command of God for the sake of their tradition. He went on to say that for the sake of their tradition, they made void the Word of God (Matthew 15:6), that they were worshiping God in vain because they were teaching as doctrine the commands of men (Matthew 15:8-9), and that they were hypocrites for setting aside the commands of God in order to establish their own traditions (Mark 7:6-9), so Jesus criticized the Pharisees for teaching their own traditions as God's Law in place of it. In Matthew 23:2-4, Jesus was not criticizing the Pharisees for teaching the people to obey what God had commanded them, but rather he was referring to all of their many traditions as placing a heavy burden on the people that they were unwilling to help lift. So the same conflict that Jesus had with the Pharisees continued between his followers and the Pharisees.
The Pharisees taught that they had the God-given authority to make rabbinic enactments, which therefore came with the same authority as a command of God, they taught that they had a higher authority than God to interpret the Torah "because the Torah is not in heaven", that therefore only they knew how to correctly obey the commands of God, and they taught that the people had to obey their oral laws in order to become saved, and this power struggle over the role of these traditions was the main issue that Paul faced, where Jews who were in power prior to Christ's ministry wanted to remain in power. This is why Paul often opened his letters by trying to establish his authority. People tend to be slow to give up their traditions, such as with Peter following an oral law in Galatians 2:11-15 (Acts 10:28), so it was an uphill battle.