An image looking east at the remains, in the foreground, of the amphitheater at Pergamum. On the left side are three arches and four piers that helped support the seating area of the amphitheater. The amphitheater was built over a river(!) and the arch on the right is one of many that were built over the river to support the arena floor.
In the distance, the acropolis of Pergamum is visible, including the cavea of the theater. To the right of the theater, the altar of Zeus was located where two pine trees are available. To the left of the theater, the white columns of the Temple of Trajan are visible.
The amphitheater of Pergamum is one of only three that are (semi) preserved in the Eastern Roman Empire (so Fairchild, p. 160). "In A.D. 157 the renowned Galen [d. ca. AD 199] was appointed as physician to the gladiators and significantly reduced the rate of death among the competitors in the Pergamon arena" (160). This is where he learned so much about human anatomy! He eventually went on to be the personal physician of the emperors Marcus Aurelius, Commodus, and Septimius Severus.
Galen may have been aware of the Christians Carpus, Papylus, and Agathonice who were martyred in the amphitheater of Pergamum during the reign of the Emperor Marcus Aurelius, ca. AD 170. It may have been their "testimony," and/or those of earlier Christian martyrs at Pergamum, that may have given him a favorable impression of this 'strange religion' in general. Larry Hurtado writes:
"Also, although Galen regarded Christianity as a defective philosophy, he expressed a certain admiration for Christians, particularly mentioning their courage in the face of death, their self-restraint in matters of sex, food, and drink, and 'their keen pursuit of justice.'" (p. 27)
Hurtado, Larry W. Destroyer of the Gods: Early Christian Distinctiveness in the Roman World. Waco, TX: Baylor University Press, 2016.
Fairchild, Mark R. Christian Origins in Ephesus & Asia Minor. Istanbul: Arkeoege, 2015, p. 160.