A.T. Robertson Commentary Luke 11:19

"And if I by Beelzebub cast out demons, by whom do your sons cast them out? therefore shall they be your judges." — Luke 11:19 (ASV)

And if I by Beelzebub (ε δε εγω εν Βεεζεβουλ). Also a condition of the first class, determined as fulfilled. A Greek condition deals only with the statement, not with the actual facts. For sake of argument, Jesus here assumes that he casts out demons by Beelzebub. The conclusion is a reductio ad absurdum. The Jewish exorcists practiced incantations against demons (Acts 19:13).