The claim comes first
Mark puts his conclusion in his first sentence, and the first man to say the whole title is a Roman officer watching him die.
This is the beginning of the gospel of Jesus Christ, the Son of God.When the centurion standing there in front of Jesus saw how He had breathed His last, he said, “Truly this man was the Son of God!”Who says it, and where
Mark opens with a headline: 'The beginning of the gospel of Jesus Christ, the Son of God' (1:1). Everything after it is the demonstration. Three voices confirm the title through the book and each one is worth noticing. God says it at the baptism, 'Thou art my beloved Son, in thee I am well pleased' (1:11), and again on the mountain, 'This is my beloved Son: hear ye him' (9:7). The unclean spirits say it and are silenced, 'Thou art the Son of God' (3:11). Then a Roman officer says it at the one place where saying it costs something, watching the man die: 'Truly this man was the Son of God' (15:39). The first human being in Mark to confess the full title is a Gentile soldier at the foot of the cross.
Mark 1:1 · Mark 9:7 · Mark 15:39

