The answer is: Isaiah

Isa 40:3

In the Gospel of Mark, one of the most striking features is how it emphasizes fulfillment of prophecy, often showing a direct pathway from Old Testament predictions to the life and mission of Jesus. When Mark quotes Isaiah at the very start, he’s setting the stage for a significant claim: that the messages of the prophets, written long ago, are not just ancient words but revelations coming to life through Jesus.

Isaiah 40:3 tells of a voice crying in the wilderness. This beautiful imagery may sound familiar if you've read the beginning of Mark’s Gospel, where John the Baptist is portrayed as that very voice. Isn't it fascinating how Mark entangles echoes of scripture straight into his narrative? John’s wilderness ministry could be viewed as a powerful invitation, mirroring the deep symbolic significance of the desert in Jewish tradition as a place of testing, purification, and divine revelation.

Trivia alert: Isaiah has a central role in Jewish prophecy because he addresses themes of comfort and hope—a move from despair to promise. And let’s not overlook that Isaiah’s writings helped shape the identity of the Jewish nation, much like the narratives in the gospels shape our understanding of Jesus!

Mark starts his Gospel with a bang, connecting Jesus’ earthly mission to the very heart of Jewish prophecy. That's one profound demonstration of continuity in scriptural history! It shows not just John’s importance but is a reminder of the promise that permeates through and across the Scriptures.