Why did Paul have to keep defending his apostleship?

Les explains that Paul had to constantly defend his apostleship because it was under constant attack, Satanic to be sure. This was part and parcel of the problems in the Corinthian church, where they had divisions: some were following what Jesus said, what Peter said, what Apollos said, and a few were staying with Paul. But they had problems because of it. Les also notes that Paul’s words are Holy Spirit inspired, not from his own feeling or bragging; the Holy Spirit inspired him to write for our benefit. Paul claimed that by the grace of God, as a wise master builder, he had laid the foundation—not that he himself was the foundation.