Based on manuscript evidence, the SBL Greek NT has dropped "the Son of God" from Mark 1:1—"the beginning of the gospel of Jesus Christ." But at his trial, when asked if he is the Messiah, the Son of God, Jesus replies "I am." (14:62)

This "I am" is very significant of course, short for "I AM WHAT I AM" (YHWH), as in John 8: 58: "Before Abraham was, I AM."

The arc from Mk 10:45 "to give his life for the ransom of many" to the centurion exclaiming "truly this was the Son of God" (15:39) is not low Christology...

So besides Paul's high Christology, Mark has one too. No development from Jesus the good rabbi to divinity, just different ways of expressing what is a mystery, or a dogma, if you will.