The answer is: Isaiah

Isa 40:3

In the Gospel of Mark, the first prophet to break ground is none other than Isaiah—a monumental figure in Scripture whose contributions resonate deeply throughout both the Old and New Testaments. This act of quoting Isaiah serves not just to highlight the prophecy, but sets a precedent for Mark’s Gospel: it emphasizes continuity within biblical narratives and draws connections between Jesus and the anticipated Messiah of Jewish tradition.

At the beginning of Mark, chapter 1, we encounter a powerful echo from Isaiah’s prophecy: “A voice of one calling in the wilderness...” Interestingly, Isaiah lived during a tumultuous time in Israel's history when the nation faced exile and despair. His role as a prophet was to prepare the way for the future restoration of God’s people, underscoring the call for repentance and redemption—a theme pivotal to Mark’s urgent and action-oriented style.

Fun trivia: Isaiah is sometimes referred to as the "fifth Gospel" because his writings—for example, the famous depiction of the Suffering Servant—are so rich with messianic predictions. Mark highlights this connection to underscore that Jesus’ ministry isn’t a sudden novelty but the continuation of God’s unfolding plan for humanity.

By kicking off with Isaiah, Mark helps us to see that what Jesus embodies is deeply rooted in a prophetic past, ensuring readers realize that the story of Jesus isn't just one of the individual, but part of a larger narrative that emphasizes hope, redemption, and God’s eternal faithfulness. Each reference builds a bridge that enlightens our understanding and appreciation of this vast tapestry.