|Lion of St. Mark, Piazza San Marco, Venice|
This passage makes two declarations about Jesus, one human and one demonic. Jesus' human auditors were amazed by him. First because of the authoritative manner in which he taught. Second, because of his authority over evil spirits. The human view of Jesus contains no hint that Jesus was anything other than a highly unusual individual, a teacher and exorcist of notable authority. It's an evil spirit that recognizes in Jesus something greater. Most versions have the demon declaring that Jesus is the "Holy One of God," but the Today's English Version (TEV) translates the Greek as "God's holy messenger." The TEV makes explicit what's implied in the other translations, namely that Jesus is of God, but subordinate to God. Jesus is "set apart" (holy). Mark, again, associates Jesus with God but doesn't state he's divine. Jesus is under the power of God and, if the TEV is a proper translation of the Greek, a prophetic figure.
It seems significant that Mark puts the declaration of Jesus' "semi-divinity" or divine associations in the mouth of a non-temporal, spirit-world being. Things about Jesus were apparent to the spirit-world that weren't evident in the human world. (This seems even more clearly expressed in Mark 1:34). In a sense, Mark here acknowledges that the gospel Jesus isn't the same as the historical Jesus. The historical Jesus was a man of authority and skill. The gospel Jesus is a man and more than a man. The author of Mark may not have had the words and concepts to express that "more-ness", but already just a generation after Jesus he was groping towards an understanding of Jesus that would ultimately make of Jesus the Second Person of the Trinity. I'd emphasize, however, that the Gospel of Mark isn't Trinitarian. Jesus, here, is fully human. He isn't fully God. The fact that Mark seems to make a distinction between human and extra-human perceptions of Jesus is hopeful from a historian's point of view. It suggests that the author probably was at least somewhat sensitive to the distinction between theological and non-theological interpretations of Jesus. He might be less likely to mask the human Jesus with a divine overlay.