The Messiah (Heb. māšîăḥ) and the Son of Man were two figures who appear in Jewish prophetic and apocalyptic writings. They later assumed great importance in Christian theology and biblical interpretation.
1. The Messiah
A number of passages scattered throughout the prophetic books of the Hebrew Bible bear witness to a hope that, at some time in the future, God would send a perfect king to rule over his people in prosperity and justice (see Is. 9.1-6, 11.1-9; Mic. 5.1-3; Jer. 23.5f, 30.9; Ez. 17.22-24, 34, 37.24f; Zech. 9.9f; and perhaps the Book of Haggai).
This king would be descended from David, Israel’s greatest past monarch, and would be a kind of David reborn. He would be a man rather than an angel or a god, though he would possess superhuman virtues and talents.
In the Old Testament, the term 'messiah' ('anointed one') means neither more nor less than 'king': it has no special connotations of the sort that it later acquired. The word is first used as a technical term for the future perfect ruler in texts outside the Bible which date to the first century BC (namely texts from the Psalms of Solomon and the Parables of Enoch).
The first century BC was the period in which messianic expectation really began to take hold in Jewish thought, though it is probably true to say that messianism was never an absolutely central tenet of Judaism. It is also true to say that ideas differed to some extent about the nature of the Messiah. Some people apparently even thought that there would be two Messiahs (we owe this insight to the Dead Sea Scrolls). On the other hand, most Jews would have had some kind of belief in the future coming of the Messiah, and most of those would have seen him as a kingly, David-like figure.
For references to the kingly Messiah in Jewish literature postdating the Old Testament, see Pss. Solomon 17.23ff, 18; Jubilees 31.18; Test. of the Twelve Patriarchs; Philo, Praem. 95; and Sibyl. Orac. 3.652-795, 5.
2. The Son of Man
In the Hebrew Bible (and in the ordinary Aramaic of Jesus’ day), ‘son of man’ meant simply ‘human being’. The term eventually came to refer to a mysterious heavenly figure who would appear at the end of time and who became conflated with the Messiah.
The transitional point can be located in chapter 7 of the Book of Daniel, which describes ‘one like a son of man’ appearing in the skies in the end times. He is a royal, triumphant figure, and seems to symbolise the righteous people of Israel. He is a transcendent being rather than a mortal human - as the text says, he is only like a son of man.
A text composed in the late first century BC and found in a document known as 1 Enoch speaks of an eschatological figure called variously ‘the Messiah’, ‘the Chosen One’, ‘the Righteous One’ and ‘the Son of Man’. He champions the cause of the righteous and passes judgement upon their enemies. He seems to be a combination of the kingly Messiah, Daniel’s Son of Man and a character known as the ‘Suffering Servant’ who is depicted in the Book of Isaiah. Like Daniel’s Son of Man, he is a transcendent being - not really human at all.
Some years later, towards the end of the first century AD, the author of 4 Ezra 11-13 drew on Daniel 7 to describe the appearance of a supernatural being ‘like the figure of a man’. Again, this being delivers the righteous and executes judgement on the sinful; and again, he is identified with the kingly Messiah.
At around the same time, the author of 2 Baruch wrote of how ‘the Messiah’ would appear or be revealed in glory and judge the Roman Empire. The writer seems, once again, to be describing a transcendent figure, and was clearly influenced by Daniel 7.
Finally, in a text composed after 70 AD known as the Apocalypse of Abraham, we read about a transcendent ‘Chosen One’ who is sent by God to rescue the righteous while their enemies are punished. He bears a strong resemblance to the supernatural Son of Man.
The idea of the Son of Man has really passed out of Jewish thought, but it is extremely important in Christian theology, since it was taken up by the early Church as a means of defining the identity and role of Jesus Christ. Jesus almost certainly used the phrase ‘the son of man’ to refer to himself, but he seems to have employed it in its original and ordinary sense of ‘human being’.