A.T. Robertson Commentary Luke 3:4

"as it is written in the book of the words of Isaiah the prophet, The voice of one crying in the wilderness, Make ye ready the way of the Lord, Make his paths straight." — Luke 3:4 (ASV)

As it is written (ως γεγραπτα). The regular formula for quotation, perfect passive indicative of γραφω.

Isaiah the prophet (Εσαιου του προφητου). The same phrase in Mr 1:2 (correct text) and Mt 3:3. Mark, as we have seen, adds a quotation from Mal 3:1 and Luke gives verses 4 and 5 of Isa. 40 not in Matthew or Mark (Luke 3:5,6). See Mt 3:3; Mr 1:3 for discussion of Luke 4:4.