The phrase "Son of Man" - υἱὸς τοῦ ἀνθρώπου - with and without the definite article (ὁ) - appears 81 times in the Gospels: 14 in Mark, 25 in Luke, 30 in Matthew and 12 in John.
- Jesus is the only one in the Gospels to use the expression to refer to Himself
- Jesus only uses this expression, and no other expression, when referring to Himself
The only passage in the Gospels where one might claim an exception is where the crowd is repeating something Jesus Himself said:
The people answered him, We have heard out of the law that Christ abideth for ever: and how sayest thou, The Son of man must be lifted
up? who is this Son of man? (John 12:34, KJV)
The late religious historian Martin Hengel made the point that although the Greek term ὁ υἱὸς τοῦ ἀνθρώπου in itself had no obvious meaning beyond the literal go a Greek speaker, the case would be altogether different when the phrase was spoken in Aramaic, as it would - as another answer hinted - immediately bring to mind the prophesy of Daniel, which appears in the Old Testament in Aramaic rather than Hebrew:1
I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him (Daniel 7:13)2
The late Protopresbyter Thomas Hopko, former Dean Emeritus of Orthodox Theology at St. Vladimir Orthodox Seminary, in his series The Names of Jesus, said:
Martin Hengel called it [the phrase, "Son of Man"] a 'veiled
codeword', a kind of Aramaic insider word for Jews. And maybe that
explains why it's found 30 times in Matthew, because Matthew's
considered to be the Aramaic Gospel,3 the one originally
written for Jewish Christians, and also, Matthew is not only the Torah
Gospel, the one identifying Jesus as the New Moses and connecting
[him] to the Law, but also Matthew is considered to be the source of
the very sayings of Jesus, that the collection of the sayings of Jesus
himself is somehow connected with the Matthew tradition and that you
find them first of all through Matthew or in a separate collection
that then comes through in Mark and Luke and so on.4
The connection with the prophesy is perhaps made even clearer in Matthew's and Mark's account of Jesus' interrogation by the high priest:
And the high priest arose, and said unto him, Answerest thou nothing? what is it which these witness against thee? But Jesus held his peace. And the high priest answered and said unto him, I adjure thee by the living God, that thou tell us whether thou be the Christ, the Son of God. Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven (Matthew 26:62-64).
And the high priest stood up in the midst, and asked Jesus, saying, Answerest thou nothing? what is it which these witness against thee? But he held his peace, and answered nothing. Again the high priest asked him, and said unto him, Art thou the Christ, the Son of the Blessed? And Jesus said, I am: and ye shall see the Son of man sitting on the right hand of power, and coming in the clouds of heaven (Mark 14:59-62).
Here He certainly brought to mind not only Daniel's prophesy, but also the Psalm:
The LORD said unto my Lord, Sit thou at my right hand, Until I make thine enemies thy footstool (110:1).
which is the most quoted Old Testament verse in the New Testament.
I think the high priest's reaction leave's little doubt as to whether the connection to the prophesy and the psalm went unnoticed:
Then the high priest rent his clothes, saying, He hath spoken blasphemy; what further need have we of witnesses? behold, now ye have heard his blasphemy (Matthew 26:65)
The question remains why Jesus chose this phrase and this allusion in the exclusive manner He did. Thomas Hopko summarizes here:
Jesus calls himself the Son of [Man] and the scribes and the elders
and the high priests, knowing the Scripture, they know exactly what he
means. They know exactly what it means.
The Greeks later ... and the Gentiles ... had to learn what it meant.
We had to learn what it meant, but a Jew did not have to learn what it
meant. He knew that that expression, “the Son of Man,” was tantamount
to the Messiah. It even is tantamount to being God’s Son. It’s
tantamount to be the one who sits on the throne, and who receives all
honor, glory, dominion, majesty, blessing, power, and thanksgiving —
evcharistia, evlogia, timē, proskēnesis — all these words are given to
him, and, certainly, to God, and to this Son of Man.
So the expression “Son of Man” is what Jesus calls himself. And it’s
the only thing that he calls himself ... Once the Son of Man—the man,
the one whom God has chosen, raised up, sent, who is his own Son
begotten of God before all ages, who already in Daniel is presented
with the Ancient of Days to sit upon the throne—once that Son of Man
comes, then he is to be given all glory, honor, and worship as the
Christ, the Lord, the Savior, the Man at the Father’s right hand, the
one who sits enthroned with him, God’s very own Son, and God himself
in human flesh.
1. Thomas Hopko, "Why does Jesus refer to Himself as Son of Man when virtually no one else does?", Ancient Faith Radio, May 30, 2009, https://www.ancientfaith.com/podcasts/namesofjesus/jesus_-_son_of_man/print (accessed 10 April 2020)
2. Septuagint reading is slightly different, where the translator includes the definite article: I beheld in the night vision, and, lo, one coming with the clouds of heaven as the Son of man, and he came on to the Ancient of days, and was brought near to him (Brenton)
3. It was the understanding in the early Church and at least through the first millennium that Matthew was the first to write a Gospel account and that he most likely wrote his first account in Aramaic (sources in translation say "Hebrew", but that is because there is no distinction between "Aramaic" and "Hebrew" in Greek). As late as the 7th century, Sophronius of Jerusalem claimed that a copy of Matthew's original work was preserved during his time at the library of Caesarea and was then being used by "the Nazarenes of Berroia in Syria" (Introduction to Theophylact's Explanation of the Gospel According to Matthew; Chrysostom Press, 1992, p.5
4. "There are four evangelists; two of them, Matthew and John, were of the company of the twelve, and two, Mark and Luke, were of the seventy. Mark was also a follower and disciple of Peter, and Luke, of Paul. Matthew, then, first wrote the Gospel in the Hebrew language for the Jews who believed, eight years after Christ's Ascension. Some say that John translated it from the Hebrew language into Greek. Mark wrote his Gospel ten years after the Ascension, instructed by Peter. Luke wrote his Gospel fifteen years after the Ascension, and John the most wise Theologian, thirty two years after the Ascension" (Theophylact, op. cit.)