Inherent in the Lord's statement is that there were Jewish exorcists at that time. He was pointing out how their own sons are accepted as casting out demons by divine power, but not the Lord Jesus? The Lord was pointing out their utter foolishness that they don't, then, accuse their own, doing the same destruction to Satan's domain, of paradoxically working for the devil, an hypocrisy that stands as an obvious testimony against them. Note apostles were doing these things, also, and this prior to Pentecost, Matthew 10:1 Jesus called his twelve disciples and gave them authority over unclean spirits so they could cast them out and heal every kind of disease and sickness. That God's Spirit worked miracles among the Jews, like casting out devils, doesn't seem unreasonable, since He was their God, too, before Christ's advent.