My Sunday work on Biblical Theology continues.
Introduction to Biblical Theology
Chapter 2: Revelation
2a From Text to Scripture
2b NT Understanding of Scripture
2c God's Speakings before Scripture
2d God's Speakings in the Old Testament
2e The New Testament Church
Chapter 3: Theology of God
3a God, the Basics
Now 3b God Almighty
... A biblical theology of God thus will organize biblical teaching both with a view to the historical and literary contexts of various Scriptures as well as to the trajectory God seems to have led his people throughout the ages. The Apostle's Creed gives us an excellent place to start as it highlights God as Father, as all powerful, and as creator. All three are clearly characteristics of God in Scripture, and we can arguably derive almost all the other key Christian theological beliefs about God from them.
God's power is a good place to begin because arguably power and immortality were the earliest and most central features of divinity in history, including the biblical text. We can perhaps see traces of God's power even in some of the earliest names of God in the biblical text. The Genesis text remembers the very early title "El Elyon," which means "God Most High," from the time of Abraham and Melchizedek (Gen. 14:18).
This title for God reveals both the polytheistic background of Abraham (e.g., Josh. 24:2) and the henotheistic nature of official Israelite religion at one point, although polytheism clearly continued (e.g., 1 Kings 18:21). Henotheism is the belief that there is only one legitimate God to worship, although other gods exist. Thus Israel must not have any gods before YHWH their God (Exod. 20:3), but there is no denial that those other gods exist. The very imagery of Psalm 82 assumes the existence of the gods of the other nations, although God Most High (82:6) is clearly far superior to them in authority and power.
As Christians we would follow Paul's lead and think of these gods more as demonic powers (1 Cor. 10:20) than as gods per se. By the later parts of Isaiah, the sole existence of YHWH as God even seems to be affirmed (Isa. 45:5).  This is a truly monotheistic position. The rise of demonology in the centuries immediately before Christ then provided a category in which to place the spiritual powers represented by the gods of the other nations.
The original sense of the Shema in Deuteronomy 6:4 is thus captured well by the NRSV translation: "Hear, O Israel: The LORD is our God, the LORD alone." Nevertheless, both Jews and Christians alike have come to understand the Shema in terms of the sole divinity of God. "Hear O Israel, the LORD our God, the LORD is one." As Christians we affirm both the original and the theological sense that Jews and Christians later heard in this verse. There is only one God, even if there may be subordinate spiritual powers.
The creation narrative of Genesis 1 may also reflect a truly monotheistic perspective. Unlike the creation stories of the Ancient Near East, creation in Genesis 1 does not involve conflict between gods in a struggle for power. Rather, Elohim (God) alone is involved and creation flows directly from his word alone. He speaks and it is done. His authority and power are unchallenged.
Christians and Jews would eventually come to believe that God created the universe out of nothing. A majority would say this belief solidified in the second century after Christ, perhaps as Jews and Christians fought off Gnosticism...
 However, even here we need to be aware that such language can be somewhat poetic. For example, is Isaiah saying that YHWH is so far superior to any other deity that, for all intents and purposes, we can say he is really the only God that truly exists? Nevertheless, a case can be made that Isaiah literally means that the other gods do not really exist at all.