The answer is: Isaiah

Isa 40:3

The Gospel of Mark opens with a powerful declaration about preparation for the coming of a significant figure, famously quoting the prophet Isaiah. This sets the tone for Mark's entire narrative, positioning Jesus' arrival not just as a singular event but as the fulfillment of ancient prophecies. Isaiah, often hailed as one of the major prophets in the Hebrew Bible, is known for his rich imagery and profound insights about justice, grace, and deliverance—concepts deeply woven into the story of Jesus.

In Mark 1:2-3, the quotation serves as a call to prepare the way of the Lord, highlighting the importance of readiness for the arrival of something transformative. Fun fact: Isaiah's prophetic ministry spanned several decades, touching upon various themes, largely during tumultuous times in Israeli history. His prophetic voice was not only about judgment but also spoke of hope—most notably in chapters that envision a time when peace and harmony would prevail.

Interestingly, this idea of preparing the way resonates through the Gospels, echoing similar sentiments found in other texts, like Malachi, leading to a vibrant expectation among the people. The mention of the wilderness in Mark also carries depth; it was in this stark, empty place that many encountered God's transformative presence. Quite fittingly, the wilderness itself becomes a symbol of both challenge and preparation—a backdrop that marks numerous biblical landmarks, from Moses' time in Exodus to Jesus' own duration of temptation. It’s all woven beautifully to demonstrate how the Great Advent transition unfolds as a saturated tapestry of historical and theological meaning.