“As it is written in Isaiah.” The quotation is from both Malachi 3:1 and the book of Isaiah 40:3. This is not “a mistake” or “error,” as some people claim, as if Mark thought the whole quotation was from Isaiah. Mark 1:2-3 are run together as if they were one quotation, not two. By just mentioning the part quoted from Isaiah, Mark is using a literary device that puts the emphasis of the extended quotation on the part that Isaiah wrote, which says what we are to do in light of the Lord’s coming. Hendrickson writes: “Mark tells us that he is going to quote from Isaiah. He does exactly that, though not immediately.” Had Mark quoted only Isaiah, we would be left knowing only that a “voice” was crying in the wilderness. By quoting Malachi before Isaiah, we know to whom the “voice” belongs: to none other than the messenger who will begin to prepare the way of the Lord.
The quotation from Malachi has been adapted to fit the Messiah. A more literal quotation of the Hebrew of Malachi 3:1 would be, “Behold, I [Yahweh] am sending My messenger, and he will clear the way [road] before me.” In Mark, the verse has been modified so that the messenger prepares the road for the Messiah. Hence here in Mark the verse means, “Look!, I am sending my [Yahweh’s] messenger before your [the Messiah’s] face, who will prepare your [the Messiah’s] way.
This is not the only place two places in the OT are quoted but only one prophet is cited. For example, Matthew 27:9-10 come from Zechariah and Jeremiah, but only Jeremiah is quoted. This same pattern occurs in the OT in 2 Chronicles 36:21, which says it quotes Jeremiah, but actually quotes both Jeremiah and Leviticus. When God quotes two sources, but only gives credit to one, He is telling us where to place the emphasis in what he is quoting so there is no guesswork about it.
In light of the fact that the extended quotation comes from Malachi and then Isaiah, it is easy to see why copyists would change “Isaiah the prophet” to “in the prophets.” The earliest texts from both the Alexandrian and Western text families have Isaiah the prophet, and the change to “the prophets” is “an obvious correction” (Roger Omanson, A Textual Guide to the Greek New Testament).
“Look!” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. See commentary on Matthew 1:20 (“Look!).
“prepare the road.” See commentary on Mark 1:3.