Of all the Gospels, Mark spends the least amount of space establishing the context for Jesus' ministry. Matthew and Luke provide lengthy accounts of the birth of Christ before recording the ministry of Jesus' forerunner, John the Baptist (Matt. 1–2; Luke 1–2). Prior to his description of the work of John the Baptist, John writes a lengthy prologue that stretches all the way back to eternity in its explicit affirmation of the Son of God's eternal preexistence (John 1:1–18). Mark, however, gives only an Old Testament quote before talking about John the Baptist (Mark 1:2–3).
But this brief quotation speaks volumes about our Lord's ministry. In beginning his gospel with a citation of the Hebrew Scriptures, Mark shows that we cannot understand the life of Jesus apart from the history of Israel. Jesus' ministry fulfills God's old covenant revelation. The new covenant community our Savior established is not wholly new; it is the continuation of the old covenant community.
Mark's source for this quote is "Isaiah the prophet" (v. 2), yet the actual quote combines Exodus 23:20; Isaiah 40:3; and Malachi 3:1. Only Isaiah is mentioned specifically because ancient Jewish writers commonly cited only the author of the most well-known text when they combined various passages as Mark has done. It is possible that Mark is not the first to combine these texts; some scholars suggest he might have taken his quote from a preexisting source that collected important messianic passages from the Hebrew Bible.
In any case, the text has a multilayered theological significance as Mark applies it to the work of John, the one who went before "the Lord." Isaiah 40:3 comes from the second half of the book of Isaiah, wherein the prophet describes a new exodus that follows the Babylonian exile. This exodus includes the final atonement for sin and is consummated in the new heavens and earth (Isa. 53; 65:17–25). In applying it to John, Mark reveals that God's salvation from our bondage to transgression and His renewal of all things takes place through Christ, whom John announced. That Mark's quote also refers to Exodus 23:20 and Moses, the leader of the first exodus, confirms that Jesus brings about a new exodus for His people. Also, we note that Isaiah 40:3 is about a voice that prepares the way for Yahweh, the one true God and covenant Lord of Israel. By applying this text to the voice that prepares the way for Jesus, Mark identifies Jesus as this one true God, implicitly teaching the deity of Christ.
Many believers neglect to study the Old Testament because they find it confusing or because they assume that it is less important to the Christian faith than the New Testament. As Mark's gospel shows us, however, this is not the case. We cannot understand Jesus or His gospel without a proper grounding in the Old Testament Scriptures. Thus, it is important for us to read and study the whole counsel of God. Let us not neglect the study of either testament.