2 Corinthians 11:26 In journeyings often, in perils of waters, in perils of robbers, in perils by mine own countrymen, in perils by the heathen, in perils in the city, in perils in the wilderness, in perils in the sea, in perils among false brethren;

A frightening prospect yet a reality in the local church – Persons who joined themselves to the Church, pretending faith in Christ, but intending to act as spies, hoping to get some matter of accusation against him. He no doubt suffered much also from apostates. [Clarke]

It is the Holy Spirit that reveals to His servants the false brethren perhaps through some giveaway in their actions, words or behaviour. The Apostle Paul was protected from such characters in his ministry work. It saved him from many hard aches that will come later if these were not exposed at critical moments.