Jamieson, Fausset and Brown on Matthew 12:24

24. But when the Pharisees heard it--Mark (Mr 3:22) says, "the scribes which came down from Jerusalem"; so that this had been a hostile party of the ecclesiastics, who had come all the way from Jerusalem to collect materials for a charge against Him. (See on Mt 12:14).

they said, This fellow--an expression of contempt.

doth not cast out devils, but by Beelzebub--rather, "Beelzebul" (see on Mt 10:25).

the prince of the devils--Two things are here implied--first, that the bitterest enemies of our Lord were unable to deny the reality of His miracles; and next, that they believed in an organized infernal kingdom of evil, under one chief. This belief would be of small consequence, had not our Lord set His seal to it; but this He immediately does. Stung by the unsophisticated testimony of "all the people," they had no way of holding out against His claims but by the desperate shift of ascribing His miracles to Satan.