The demons possessing the men knew that Jesus is the Son of God (Luke 4:41) because of supernatural understanding, like Satan himself (Matthew 4:1–6). The phrase and behold
shows that their crying out their knowledge occurred suddenly. The demons know more than the disciples, but it is a fearful knowledge, because they know that Jesus’ coming will lead to them being cast into the abyss (Luke 8:31, Revelation 20:2–3)
Being supernatural beings, these demons, just like their lord the devil did earlier (Matthew 4:1–6), immediately recognize Jesus as the Son of God, the Messiah (Luke 4:41). The demon-possessed men therefore start to cry out to Jesus, with the introductory phrase and behold
indicating that it happened quite suddenly. The evil spirits possessing them know that with the arrival of the Messiah and the coming of the kingdom of God, they will be cast into the abyss (Luke 8:31, Revelation 20:2–3).
When these two demon-possessed men publicly acknowledge Jesus as Son of God,
they were acknowledging a truth which Jesus at the time had not yet revealed even to the apostles, who were not yet ready to confess him as such. At the time they merely acknowledged that he was an exceptional man.1
29 And behold, they cried out, “What have you to do with us, O Son of God? Have you come here to torment us before the time?”