This is a title that Jesus uses for himself in the gospels, and it has a complex background. Since it has not been widely used by Christians, it can be helpful to look at its earlier uses.
The Greek ho huios anthropou in the Gospels translates an Aramaic or Hebrew original bar nash. The Greek has two words that have been translated ‘man’ in English, but they differ in meaning. [Hebrew is similar.] One is for a male, but anthropou means any ‘human’. (It is the root of such English words as anthropology and philanthropist.) This is harder to translate as the Jesus title, lacking a suitable equivalent – ‘Son of Humanity’ is not quite the same. ‘Son of…’ expressions seem to be a figure of speech; others in the gospels are ‘sons of thunder’ and ‘son of perdition’ which have a poetic colour. There is a Semitic grammar here that we cannot duplicate in English.
The first meaning in the time of Jesus for Son of Man is that it is simply a synonym for a person, ‘a human being’, as in Ps 8:6 where it is in parallel (in the manner of Hebrew poetry). ‘What is a human that you should notice ‘him/her’ or the son of a human that you should care for ‘him/her’?’ In the Book of Ezekiel the prophet is addressed by the Lord as ‘son of man’ –about 87 times – where again it means a human person. The word was also used as a roundabout reference to oneself, and that may be the way it was heard by those around Jesus.
There is no recorded use of the phrase as a messianic title until after first Jewish war, ad 70-100, contemporary with gospels; it is found in late writings not part of our Bibles, 4 Ezra 13, 1 Enoch 37-71, and in Rabbi Akiba, before 135. This use for ‘The Messiah’ was based upon a passage in the Book of Daniel which is an ‘apocalypse’ masquerading as a prophecy. In a night of visions, Daniel reports seeing that four kingdoms that had oppressed the Jews – which he poetically called ‘beasts’ – will be destroyed and the people freed. Then there is a vision of the ‘Ancient of Days’ (as traditionally translated) which is God sitting in judgement. After the last beast is destroyed, Daniel says, ‘I saw, coming on the clouds of heaven, one like a son of man. He came to the ‘Ancient of Days and was led into his presence. On him was conferred sovereignty, glory and kingship, and all peoples of the earth, nations and languages became his servants. His sovereignty is an eternal sovereignty which shall never pass away, nor will his empire ever be destroyed.’ This vision of all power conferred on ‘the Son of Man’ fit into the idea of a Messiah whose rule will everlasting.
The question for us as readers is what it meant for Jesus and Mark. Since Jesus uses it for himself, what does he mean by it? E. P. Sanders has this advice: ‘We do not learn precisely what Jesus thought of himself and his relationship to God by studying titles….there were no hard definitions of ‘Messiah’, ‘Son of God or ‘Son of Man’ in the Judaism of Jesus’ day. Even if he had constantly called himself by all three titles, we could learn what he thought of himself only by studying him – not by studying the titles in other sources.’
In its first appearance in Mark, 2:10, Jesus as Son of Man claims power to forgive sins on earth, and demonstrates it by a healing. Another later use claims that he is Lord of the Sabbath. A number of times the title will be in relation to his death and resurrection.