Loren T. Stuckenbruck on Exorcisms in the Gospels by non-believers in Christ

[I]t is significant that successful exorcisms in the Synoptic Gospels are not only attributed to Jesus alone. Not only are exorcisms performed by Jesus’ disciples (Mk. 6:7, 13-15; Mt. 10:1, 7-8; Lk. 9:1; 10:17-20) , they are also a condonable activity practiced by those who are not among Jesus' immediate followers. This is, for instance, the case with the so-called "strange" exorcist mentioned in Mar 9:38-41 (par. Lk. 9:49-50) as well as with Jesus' rhetorical question which assumes that his Jewish contemporaries were likewise able to exorcise demons (Mt. 12:27 par. Lk. 11:19: "by whom do your sons cast out?"). The latter episode, which is tucked away in a passage that focuses on Jesus' response to an accusation that questioned the source of his power, admits that an analogy exists between the exorcisms of Jesus and those performed by the "sons" of his interlocutors (the "Pharisees" in Matthew and the "scribes" in Luke). In other words, Jesus' encounters with the demonic world are located by the Synoptic Gospels within a religious climate in which exorcisms were considered a legitimate, if not effective way to combat evil. According to the Gospels, Jesus, as an exorcist, participated in a worldview in which exorcism makes sense For all the wish of the Gospel writers to accentuate Jesus' prowess and expertise in this area, this portrait of Jesus would have shared some fundamental assumptions with his contemporaries on how such activity works and what it signifies for both practitioners and those deemed to be under demonic sway. Thirdly, both the multiple attestation in the Gospels and their recognition of exorcism as an effective practice among non-devotees of Jesus strengthen the likelihood that we are dealing with the shared preservation of an early tradition that may have been circulating during the time of Jesus' ministry/ Whether or not they identified with the Jesus movement, a wide number of contemporaries believed that Jesus engaged in open conflict with demonic beings. However much individual pericopae, especially the exorcism episodes, were shaped by conventional oral and literary forms, there is no reason to doubt that any reconstruction of Jesus' life and ministry that does not include the claim that he expelled evil spirits omits something essential. Likewise, since each of the Gospel writers wished to emphasize the unprecedented magnitude of Jesus' life, teaching, and ministry, there is no compelling reason why a post-Easter community would have generated stories that acknowledge the performance of exorcisms by his contemporaries. (Loren T. Stuckenbruck, The Myth of Rebellious Angels: Studies in Second Temple Judaism and New Testament Texts [Grand Rapids, Mich.: Eerdmans, 2017], 170)