In Mark 1:24, a demon tells Jesus "I know who you are—the Holy One of God!" This title is surprising since the title "the Son of God" is used to introduce Jesus in 1:1. Obviously the title denotes Jesus' holiness here, but what connotations would it have had? Is it a kingly title? Priestly title? Divine title?
In the New Testament it's used in the parallel passage in Luke 4:34, but also in John 6:69 in a passage reminiscent of Peter's confession in Mark 8 of Jesus as the "Son of God." But I couldn't find other usages of the title in a wider search. Was this a known title at the time? If so, what are its roots?