The quotations in Mark 1:2-3 are from Isaiah 40:3 and Malachi 3:1. Thus, the correct reading should be “the prophets.” Critics in support of the "Isaiah" reading point that it is supported by the oldest extant Greek uncial Codex Sinaiticus. However, Sinaiticus inserts “Isaiah” in Matthew 13:35, making the verse read, "that it might be fulfilled that was spoken through the prophet Isaiah, saying: I will open my mouth in parables, I will utter things concealed from the foundation." However, the quote is from Psalm 78:2, not Isaiah. “Isaiah” in Matthew 13:35 of Sinaiticus is so obviously an error that all Bibles abandon the oldest manuscript, and follow the reading of later manuscripts. Seeing that Sinaiticus inserted “Isaiah” in Matthew 13:35, one could reasonably suspect that an Alexandrian scribe also inserted “Isaiah” in Mark 1:2. The reading, "the prophets" in Mark 1:2 is in Codex Alexandrinus of the 5th century and also quoted by Iranaeus in the 2nd century (Against Heresies III 10:5).
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