If we just forget about the capitals for one moment, the expression son of (a) god is almost as old as writing (> 5000 y.). To understand it we have to get back, not only upstream of Christianity, but also upstream of the constitution of monotheism in the last 6 centuries before the birth of Christianity.
We can then discern at least three related meanings, which we will have to replace in their original context and in the context of their possible reception at the beginning of the Christian era.
1. mythological: in Semitic languages, a son of (a) god is basically a god ? just as a son of (a) man is a man. The expression originally belongs to a polytheistic context, which is still apparent in a number of Biblical texts:
Psalm 82:1,6 God (= Yhwh) has taken his place in the divine council; in the midst of the gods he holds judgment. (...) I say, "You are gods, sons of the Most High (Elyôn, another designation of the supreme god El), all of you. Psalm 29:1 Ascribe to Yhwh, O sons of gods, ascribe to Yhwh glory and strength. Psalm 89:6f For who in the skies can be compared to Yhwh? Who among the sons of gods is like Yhwh, El feared in the council of the holy ones, great and awesome above all that are around him? Dt 32:8 (corr.) When the Most High apportioned the nations, when he divided humankind, he fixed the boundaries of the peoples according to the number of the sons of El (Yhwh, one of them, inherits Israel in v. 9). The sons of the god(s) in Genesis 6:1-3 clearly belong to this polytheistic context, which is still somewhat reflected in Job 1:6 etc.
This original meaning, although repressed in Judaism with the conversion to monotheism, remains in the 1st-century AD a very present possibility in Graeco-Roman mythology: the gods beget gods, semigods and heroes like Dionysos or Herakles (whose analogies with Christ will be explicitly used by later Christian apologetics). Moreover, ?divine men? (theioi andres) can also be called sons of god(s).
When moving from polytheism to monotheism, Judaism came to understand the expression as meaning « son(s) of God », i.e. heavenly being(s) subject to the unique God: ?angels?, good and bad according to the context. So the Greek Septuagint often translates son(s) of (the) god(s) as angels of God. In this perspective the idea of one main, archetypal heavenly Son of God develops, as a sort of chief-angel (or arch-angel). He may be seen as eternal, though not equal to God (cf. Mc 13,22); this tradition merges with another, that of the Wisdom personified/hypostatized as ?daughter of God? (from Proverbes 8 on). At the beginning of the first century, the Alexandrine Jew Philo often mentions such a character, to whom he attributes both angelic and wisdom features:
De Confusione linguarum 146: ?And even if there be not as yet any one who is worthy to be called a son of God, nevertheless let him labour earnestly to be adorned according to his first-born word, the eldest of his angels, as the great archangel of many names; for he is called, the authority, and the name of God, and the Word, and man according to God?s image, and he who sees ? Israel.? 63 « For the Father of the universe has caused him to spring up as the eldest son, whom, in another passage, he calls the firstborn (prôtogonos). And he who is thus born, imitating the ways of his father, as formed such and such species, looking to his archetypal patterns.?
2. political: the son of the god (either the tutelar god of the nation, or the supreme god, often the sun-god) is the king (who is also the high priest, cf. Psalm 110:4), himself logically deemed an incarnate god. This conception is ubiquous in the Ancient Near East, from Egypt to Mesopotamia, including Syria and Phoenicia. Israel is no exception: the proclamation of the king?s divine begetting is an essential part of the (perhaps yearly commemorated) enthronement ceremony:
Psalm 2:6f "I have set my king on Zion, my holy hill." I will tell of the decree of Yhwh: He said to me, "You are my son; today I have begotten you.? Psalm 110:3 (corr.) From the womb of dawn (cf. Es 14:12 where the king of Babylon is called son of Dawn, which is the name of a goddess [Shahar]), like dew I begot you.? (There are important Ugaritic parallels for the mythological overtones of this text.) (2 Samuel 7:14 I will be a father to him, and he shall be a son to me. // 1 Chronicles 17:13; 22:10; 28:6 + 4QFlor 11-13).
The king is also « son of god » in the strong sense of « god » (at this stage there is no weaker meaning in Hebrew). So he may also be called ?god?:
Psalm 45:6f, hymn to the king (v. 2): Your throne, O god, endures forever and ever. Your royal scepter is a scepter of equity; you love righteousness and hate wickedness. Therefore, O god, your god has anointed you with the oil of gladness beyond your companions. Psalm 89:27f He shall cry to me, 'You are my Father, my god, and the Rock of my salvation!' I will make him (the king) the firstborn, the Most High (divine title) of the kings of the earth. Isaiah 9:5: the kingly child is called a mighty god.
This old political use of the expression acquires a new actuality in the polytheistic context of the Roman empire, where the Emperor is proclamed (in latin) divi filius, « son of the divine » -- i.e., son of a former emperor divinized after his death. The ruling emperor may be such a divinized emperor?s biological son (so Titus and Domitian, sons of Vespasian) or adopted son (Octavius-Augustus adoptive son of Julius, Tiberius adoptive son of Octavius, Nero adoptive son of Claudius). In the Eastern empire, this title is translated in Greek theou huios, ?son of (a) god?, a ubiquous formula (appearing on stelae and coins) which is exactly found in Matthew (14:33; 27:43.54). The enthronement of the emperor is an evangelion, ?gospel? or ?good news?.
When shifting from the political context of a divine right monarchy to the situation of a province (and diaspora) in a « pagan » world empire, the « son of god » statute assigned to the king in the ancient religion of Israel was reported by mainstream Judaism upon a future, apolyptical character, which tends to become universal inasmuch as it is conceived as a kind of anti-emperor. What we usually call (although it is everything but a clear concept before Christianity) ?the Messiah?. This character combines kingly and priestly features (in the old times the king was the high priest, and the last kings in Judea were the Hasmonean high priests, regardless of the intervening Levitical law separing the two functions).
As was mentioned in Peacefulpete's thread, this title appears in a similar sense in a recently translated Qumran manuscript, 4Q246: He shall be hailed as Son of God, and they will call him Son of the Most High. It is also used in other messianic texts of Jewish apocalyptics ( 4 Ezra 7:28f my son, the Messiah; 13:32,37,52, the revelation of ?my son?; 14:9: Ezra will be united in death ?with my Son and with those like you until the times are fulfilled?).
3. ethnical/ethical: in a number of ancient texts, the son of the god is his people (collective singular):
Exodus 4:22f: Israel is my son, my firstborn; cf. Jr 31:9,20 (Israel / Ephraim); Os 11:1 Deuteronomy 32,6.18 Is not he your father, who created you, who made you and established you? (...) You were unmindful of the Rock that bore you; you forgot the God who gave you birth. Jeremiah 3:4,19 calls Israel?s god « Father ». This still occurs in Wisdom 18:13: when their firstborn were destroyed, they acknowledged your people to be God's son.
- or its members (plural) -- Deuteronomy 14:1; 32:5,19; Isaiah 1:2; 30:1; 43:6; 45:11; Jr 3:22; Ez 16:20s (God is their Father and the nation is their mother); Hosea 2:1. « Our father » develops as an invocation, Isaiah 63:16; 64:7; Ml 2:10
This « son(s)-statute » attributed to the people is often connected to moral exhortation, but hardly as a condition. Unworthy sons do not automatically cease from being sons; even when there is a threat of disowning, there is also the prospect of restoration.
This may be seen as a variant for the famous doctrine of ?election?, the meaning of which radically changes with the shift from polytheism to monotheism. From the old affirmation ?the tutelar god chooses, adopts or begets his people? a quite different assertion is made: ?the only God chooses, adopts or begets one people as his own?.
Were monotheist Jews uneasy before such an arbitrary concept? Remarkably, with monotheism, ethics come to the fore. The ?son of God? (again singular, as an exemplary character) become the righteous. In a Palestinian context, this development first appears as a narrowing of the ethnical meaning (being Jewish is not sufficient for being a son of God):
In Jubilees 1:24, righteousness implies fulfilling the Torah: Their souls will cling to me and to all my commands, they will fulfill my commands, I will be their Father and they will be my sons. Siracides 4:10: Be a father to orphans, and be like a husband to their mother; you will then be like a son of the Most High, and he will love you more than does your mother. (Hebrew: God will call you his son, he will extend grace to you and save you from destruction.) (Cf. the invocation « Father » in 23,1.4; 51,10.)
But paradoxically this development also broadens the potential scope of the expression (what matters is being righteous, not being Jewish): this wider perspective is obvious in Hellenistic diaspora literature, especially in Philo.
A particularly noteworthy text is Wisdom of Solomon, which was written shortly before Jesus? time; e.g. 2:13ff:
He professes to have knowledge of God, and calls himself a child of the Lord. He became to us a reproof of our thoughts; the very sight of him is a burden to us, because his manner of life is unlike that of others, and his ways are strange. We are considered by him as something base, and he avoids our ways as unclean; he calls the last end of the righteous happy, and boasts that God is his father. Let us see if his words are true, and let us test what will happen at the end of his life; for if the righteous man is God's child, he will help him, and will deliver him from the hand of his adversaries. Let us test him with insult and torture, so that we may find out how gentle he is, and make trial of his forbearance. Let us condemn him to a shameful death, for, according to what he says, he will be protected.
Then the righteous will stand with great confidence in the presence of those who have oppressed them and those who make light of their labors. When the unrighteous see them, they will be shaken with dreadful fear, and they will be amazed at the unexpected salvation of the righteous. They will speak to one another in repentance, and in anguish of spirit they will groan, and say, "These are persons whom we once held in derision and made a byword of reproach--fools that we were! We thought that their lives were madness and that their end was without honor. Why have they been numbered among the children of God? And why is their lot among the saints?This text had an obvious influence on the Gospel Passion narratives ? remember the centurion?s confession: « this man was (a) son of God » (Mark 15:39, cf. 1,1 ou 11 // Matthew 27,54) -- « a righteous one », Luke 23:47 translates accurately (although losing sight of some theological overtones).
The ?ethical? use of the expression is not exclusive. In Jesus? sayings (some of them being really "pre-christian") the title ?son of God? is applied in the same way, as a consequence of a moral or supra-moral background, regardless of any relationship with Jesus himself:
Matthew 5:9 Blessed are the peacemakers, for they will be called sons of God. 5,45// (Love your enemies) so that you may be sons of your Father in heaven; for he makes his sun rise on the evil and on the good, and sends rain on the righteous and on the unrighteous.
In the 1st century AD the three lines of meaning converge : the persecuted righteous, exalted as God?s son, is glorified in terms reminiscent of the heavenly angelic ?sons of God?, or even of the ?Only? or ?firstborn Son?. This is apparent in the literary developments concerning the patriarchs (Abel, Enoch, Abraham, Jacob-Israel, Levi, Joseph, Moses) who are described as exalted to heaven. For instance:
In the Abel section of the Testament of Abraham (12?13): on this throne an amazing man was sitting, radiant as the sun, like a son of God (12:6). Testament of Levi 4:2 : The Lord heard your prayer ? to be separated from unrighteousness and to be a son and a servant to him, and a minister to His face. In Joseph and Aseneth, Joseph is described as the son of God (6:2,6; 13:10) and God?s firstborn son (21:3).
At the same time the apocalyptical character of the Messiah-King Son of God tends to be identified with the same heavenly figure, inasmuch as his fulfillment within the realm of politics fades out from realistic expectation.
So much for the background of the expression in the first century. How it was actually (and diversely) received and applied to Jesus in early ?christianities? may be the subject for another post, if this one raises any interest/discussion.