Jamieson, Fausset and Brown on Revelation 2:12

12. Trench prefers writing Pergamus, or rather, Pergamum, on the river Caicus. It was capital of Attalus the Second's kingdom, which was bequeathed by him to the Romans, 133 B.C. Famous for its library, founded by Eumenes (197-159), and destroyed by Caliph Omar. Parchment, that is, Pergamena charta, was here discovered for book purposes. Also famous for the magnificent temple of Æsculapius, the healing god [Tacitus, Annals, 3.63].

he which hath the sharp sword with two edges--appropriate to His address having a twofold bearing, a searching power so as to convict and convert some (Re 2:13, 17), and to convict and condemn to punishment others (Re 2:14-16, especially Re 2:16; compare also see on Re 1:16).