Mark's opening line
Whether 'Son of God' belongs to Mark's first sentence or entered it from the tradition.
Mark 1:1
1The beginning of the Good News of Jesus Christ, the Son of God.
What each tradition says
Rabbinic / Midrashic
Kohler and Hirsch argue that the Synoptic Gospels show Jesus never called himself the son of God in any sense beyond the righteous calling themselves sons or children of God. The parable of the faithless husbandmen and vineyard (Mark xii. 1) does not support the assumption that Jesus described himself as “son of God” in a specific theological sense.
Kaufmann Kohler and Emil G. Hirsch, The Jewish Encyclopedia (1905), art. "Son of God" — On the Synoptic Gospels (the title's sense on Jesus' lips)
Patristic
Irenaeus says Mark, the interpreter and follower of Peter, begins his Gospel narrative with “The beginning of the Gospel of Jesus Christ, the Son of God,” followed by “as it is written in the prophets, Behold, I send My messenger before Thy face, which shall prepare Thy way.” So Irenaeus places “Son of God” in Mark’s opening words.
Irenaeus of Lyons, Against Heresies (Ante-Nicene Fathers, vol. 1; trans. A. Roberts and W. Rambaut) — Book III, ch. 10, §5 — Mark's opening words
Catholic
MacRory holds that the dominant note of the whole Gospel is sounded in the first verse: “The beginning of the gospel of Jesus Christ, Son of God.” He says Westcott and Hort remove “Son of God” from the text, but quite improperly, citing Knabenb., “Comm. in Marc.,” 23.
J. MacRory, The Catholic Encyclopedia (New York: Robert Appleton Company, 1910), art. "Gospel of St. Mark" — On the first verse (the words 'Son of God')
Orthodox
No sourced position found. Checked CCEL ANF/NPNF (no Greek commentary on Mark in English; the Mark passage cited comes from Irenaeus, a Latin father), archive.org search for a public-domain English Theophylact/Euthymius on Mark (only the modern Chrysostom Press translation), pravoslavieto.com (Philaret's Catechism; Synod of Jerusalem 1899 — nothing on Mark i. 1's text).
Reformed / Protestant
Gill says Mark begins his Gospel with the divine sonship of Christ, as Paul began his ministry with it. Matthew began with Christ's humanity, calling him son of David; Mark begins with his divinity, calling him Son of God. Christ is son of David in human nature and Son of God in divine nature, united in one person.
John Gill, John Gill's Exposition of the Whole Bible — On Mark 1:1 (Mark's opening with the divine sonship)
Evangelical
Albert Barnes says the title “Son of God” was used here to attract attention and secure the respect of those who would read the Gospel. It shows this is no common history, not recounting the deeds of a man, hero, or philosopher, but the doctrines and doings of the Son of God.
Albert Barnes, Albert Barnes' Notes on the Bible — On Mark 1:1 (the title 'Son of God')