A.T. Robertson Commentary Mark 1:2

"Even as it is written in Isaiah the prophet, Behold, I send my messenger before thy face, Who shall prepare thy way." — Mark 1:2 (ASV)

In Isaiah, the prophet (εν τω Εσαια τω προφητη). The quotation comes from Mal 3:1 and Isa 40:3. The Western and Neutral classes read Isaiah, the Alexandrian and Syrian, "the prophets," an evident correction because part of it is from Malachi. But Isaiah is mentioned as the chief of the prophets. It was common to combine quotations from the prophets in testimonia and catenae (chains of quotations). This is Mark's only prophetic quotation on his own account (Bruce).