The Son Of Man
I've encountered many discussions where Christians assume that calling Jesus "the Son of Man" means God incarnated as a man or the human part of God. But then the Unitarians may point out:
"God is not a man, that He should lie, nor a son of man, that He should repent. — Numbers 23:19 (NKJV)
Common Meaning
In general "son of man" refers to any general human, for example:
“What is man that you are mindful of him, and the son of man that you care for him?” — Psalm 8:4 (ESV)
See also Psalm 80:17, 144:3, 146:3.
This is evident from the book of Ezekiel where Ezekiel is repeatedly called "the son of man", for example:
He said to me, “Son of man, stand on your feet, and I will speak with you.” — Ezekiel 2:1 (ESV)
Job likewise uses it in an ordinary human sense:
“Your wickedness concerns a man like yourself, and your righteousness a son of man.” — Job 35:8 (ESV)
See also Job 16:21 and Job 25:6 as well as the prophets like Isaiah 51:12, 56:2; Jeremiah 50:40, 51:43; Daniel 8:17; etc.
The lexical background matters too. The Hebrew adam, often rendered “Adam”, also means man. Genesis presents Adam as man, and Genesis 3:15 promises the woman's offspring. That promise is a possible or thematic link to Jesus as the human Messiah, not a statement that God himself would become the offspring.
However, when applied to Jesus, it links back to Old Testament prophecies.
Prophecies
Son of Adam
In Genesis the same Hebrew name that is often translated as "Adam" also means "man". For example:
So God created man in his own image... — Genesis 1:27 (ESV)
Most Bible translations continue to use the term "the man" in Genesis 2 until Eve was created.
God gave His word to Eve:
I (God) will put enmity between you (the devil) and the woman (Eve), and between your offspring and her offspring;
He shall bruise your head, and you shall bruise his heel.”
— Genesis 3:15 (ESV, with interpretive notes added for clarity)
The text shows that God did not tell Eve that He Himself would personally come and bruise the devil. Instead, He promised that Eve's offspring would bruise the devil's head. Because this offspring could be considered the son of Adam (man) it potentially refer to Jesus would accomplish.
A possible interpretations is that this was God's word that given in the beginning of which John 1 wrote. In that context the "He" in Genesis 3:15 would refer to Jesus.
Daniel's Prophecy
Daniel 7 is the title's royal source. One like a son of man comes with the clouds before the Ancient of Days and receives kingdom, glory, dominion, and judgment authority. These cloud, kingdom, glory, and judgment texts belong to Daniel's category.
“I saw in the night visions, and behold, with the clouds of heaven there came one like a son of man, and he came to the Ancient of Days and was presented before him. And to him was given dominion and glory and a kingdom, that all peoples, nations, and languages should serve him; his dominion is an everlasting dominion, which shall not pass away, and his kingdom one that shall not be destroyed. — Daniel 7:13-14 (ESV)
The vision distinguishes the one like a son of man from the Ancient of Days who gives him this rule. Daniel therefore supplies both a human designation and a royal messianic setting.
This "son of man" is not just another "human", but is often referred as the Messiah (Christ) who would be God's glorious King.
Matthew 24:30, 26:64; Mark 13:26, 14:62; Luke 21:27 applies Daniel's the prophecy to Jesus Christ identifying the "clouds of heaven" which Daniel also identified.
Isaiah's Prophecy
Isaiah 52:13–53:12 supplies a different category: the servant's service, suffering, death or ransom, and exaltation. Isaiah 53 does not use the title “son of man”. It describes the servant theme, not the title's source.
Behold, My servant shall act wisely;
he shall be high and lifted up, and shall be exalted.
He was despised and rejected by men,
a man of sorrows, and acquainted with grief.
But he was pierced for our transgressions;
he was crushed for our iniquities;
upon him was the chastisement that brought us peace,
and with his wounds we are healed.
And the LORD has laid on him
the iniquity of us all.
By oppression and judgment he was taken away;
and as for his generation, who considered
that he was cut off out of the land of the living,
stricken for the transgression of my people?
Out of the anguish of his soul he shall see and be satisfied;
by his knowledge shall the righteous one, My servant,
make many to be accounted righteous,
and he shall bear their iniquities.
Therefore I will divide him a portion with the many,
and he shall divide the spoil with the strong,
because he poured out his soul to death
and was numbered with the transgressors;
yet he bore the sin of many,
and makes intercession for the transgressors.
— Isaiah 52:13–53:12 (ESV)
Isaiah prophecy predicted God "servant" would match Jesus' life.
Matthew records:
... the Son of Man came not to be served but to serve, and to give his life as a ransom for many.” — Matthew 20:28 (ESV); Mark 10:45
Jesus Identity
When Jesus asked the question:
"Who do people say that the Son of Man is?" — Matthew 16:13 (ESV)
The Jews did not answer "God as a man", but instead...
they said, “Some say John the Baptist, others say Elijah, and others Jeremiah or one of the prophets.” — Matthew 16:14 (ESV)
The Jews expected that "the Son of Man" means one of their prophets instead of an incarnated god.
Jesus Authority
Many times when Jesus refers to himself as "the Son of Man", he uses it in the context of God's suffering "servant", for example:
For just as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth. — Matthew 12:40 (ESV)
And as they were coming down the mountain, Jesus commanded them, “Tell no one the vision, until the Son of Man is raised from the dead.” — Matthew 17:9 (ESV)
See, we are going up to Jerusalem. And the Son of Man will be delivered over to the chief priests and scribes, and they will condemn him to death...even as the Son of Man came not to be served but to serve, and to give his life as a ransom for many — Matthew 20:18,28 (ESV)
Sometimes Jesus as "the Son of Man" is used in the context of his God given authority as the Christ (Messiah), for example:
But that you may know that the Son of Man has authority on earth to forgive sins — Matthew 9:6 (ESV)
For the Son of Man is lord of the Sabbath — Matthew 12:8 (ESV)
For the Son of Man is going to come with his angels in the glory of his Father... — Matthew 16:27 (ESV)
... the Son of Man coming in his kingdom — Matthew 16:28 (ESV)
It also sometimes refer to Jesus' judgement, for example:
For as the lightning comes from the east and shines as far as the west, so will be the coming of the Son of Man... then will appear in heaven the sign of the Son of Man, — Matthew 24:27,30 (ESV)
Therefore you also must be ready, for the Son of Man is coming at an hour you do not expect. — Matthew 24:44 (ESV)
The list is incomplete and there are many such references about Jesus in the Gospels.
Jesus Compared To Adam
Although Paul does not use the title “Son of Man”, he does call Jesus “a man” and sets him as the representative counterpart to Adam:
For as by a man (Adam) came death, by a man (Jesus) has come also the resurrection of the dead.
...
Thus it is written, “The first man Adam became a living being”; the last Adam became a life-giving spirit. But it is not the spiritual that is first but the natural, and then the spiritual.
The first man was from the earth, a man of dust; the second man is from heaven. As was the man of dust, so also are those who are of the dust, and as is the man of heaven, so also are those who are of heaven. Just as we have borne the image of the man of dust, we shall also bear the image of the man of heaven. -- 1 Corinthians 15:21,45-49 (ESV)
Nobody ever though Adam was a god and comparing a god with a man would have been pointless.
Although Paul stated that both Adam and Jesus were different from us:
Adam, who was a type of the one who was to come. — Romans 5:14 (ESV)
Adam was a pattern or foreshadowing of Jesus. Both were unique humans with special origins and purposes.
This comparison supports Paul viewing Jesus as human: Adam's representative failure is answered by Jesus' representative work. The possible Genesis 3:15 offspring theme supports this reading.
Conclusion
“Son of Man” means human in its ordinary biblical use.
Applied by Jesus, it is also Daniel's royal messianic title, while his service, suffering, death, ransom, and exaltation fit Isaiah's servant theme.
Paul’s Adam comparison calls Jesus a human representative counterpart to Adam.
None of these makes the title a name for God or proof of divine incarnation. Jesus was not an angelic being, nor divine superhuman, but a genuine 100% human like one of us. This is not to be confused with his role and authority which are clearly above any other human leader that ever lived.
Jesus is the most important king and he still reigns.