Mark includes Old Testament prophecies (messages from God given long ago) to support his account of the good news about Jesus Christ. John the Baptist’s role as a prophet and messenger shows that Jesus is the promised Lord and king. He is the Christ (or Messiah, God’s chosen one) and the Son of God.
Isaiah: Mark follows the Jewish practice of mentioning only the best-known source in a quotation. Here he names Isaiah, even though the quotation also includes (Malachi 3:1; Isaiah 40:3).