Many modern commentators deny the divinity of Jesus, arguing that He never claimed to be God. However, the book of Mark declares at the beginning, in what serves as a title, “The Beginning of the Good News about Jesus the Messiah, the Son of God” (Mark 1:1).
“Beginning” may refer to the beginning of the book, or it may refer to the idea that Jesus’ earthly ministry was only the beginning of the Kingdom of God. Acts 1:1 says that the Gospel of Luke described “all that Jesus began to do,” implying that Jesus would continue to work, through the church, as described in the book of Acts (and, by extension, through the church in subsequent centuries).
The title also makes clear that Jesus is the Messiah, tying Jesus in to all that God had revealed in the Old Testament. This is clear, first, in the name “Jesus,” which means “Yahweh saves”—Yahweh is the name by which God revealed himself in the Old Testament. The “Messiah” was the coming Savior prophesied throughout the Old Testament. Mark 1:2-3 then cites the words of “Isaiah the prophet” as an example: “I will send my messenger ahead of you, who will prepare your way—a voice of one calling in the wilderness, ‘Prepare the way for the Lord, make straight paths for him.’” This is actually a compound quote from Exodus 23:20, Malachi 3:1, and Isaiah 40:3. Mark is obviously referring to John the Baptist, but there is a sense in which not just John the Baptist but all of the Old Testament is a forerunner to Jesus.
Then the writer of Mark (presumably John Mark, reflecting the teaching of Peter) states plainly that Jesus is “the Son of God.” Now, Peter did not understand this fully at the beginning of Jesus’ ministry—Jesus only revealed this information slowly—but by the end of Jesus’ life on earth, this fact was absolutely clear. Later commentators may not like the claim, but it is one made clearly by Jesus and by His followers.