Among false brethren; false teachers and private persons, who corrupted the Christian religion, and were as great enemies to the apostle as any he had.
Gill's Exposition of the Entire Bible
In journeying often,.... Through several countries and kingdoms to preach the Gospel, as he did from Jerusalem round about to Illyricum:
in perils of waters; by the floods being out, which made it very troublesome and dangerous travelling, especially to persons on foot, as was the case of our apostle:
in perils of robbers; for though he had seldom much to lose, yet was in danger of being ill used, and of his life being taken away by such ruffians:

in perils by my own countrymen; the Jews, who bore an implacable hatred to him, because of the doctrines of grace he preached, in opposition to the works of the law of Moses, whether moral or ceremonial; and who generally were concerned in stirring up the Gentiles against him wherever he came:

in perils by the Heathen; the Gentiles, who were incensed against him for inveighing against their idols and idolatrous worship, and other wicked and enormous practices they were addicted to; particularly at Ephesus, by the means of Demetrius the shrine maker, Acts 19:23,

in perils in the city; in any and every city he came into; for bonds and affliction abode him everywhere, as at Jerusalem, Damascus, Antioch, Ephesus, Philippi, Thessalonica, &c.

in perils in the wilderness; by robbers and wild beasts, through hunger and thirst, and by the sands in hurricanes and tempests; though this may be understood not strictly of desert places, but of the country in distinction from the city; See Gill on Matthew 3:1 where travelling is difficult and dangerous, and the people more rustic and uncivil: the phrase, , "in perils of the wildernesses", is a Rabbinical one (p); as is also , "peril in the sea" (q), next mentioned: https://biblehub.com/commentaries/2_c...