In his doctrine - In his “teaching,” for so it should be rendered.
Beware of the scribes - Be on your guard. Be cautious about hearing them or following them.
Scribes - The learned men of the Jewish nation.
Which love to go in long clothing - In long, flowing robes, as significant of their consequence, leisure, and learning.
Salutations ... - See the notes at Matthew 23:6-7.