While images featured in the cult of Yahweh (e.g. the cherubim), and Yahweh appeared in visions to the prophets, who then recorded those visions in books (e.g. Yahweh surrounded by seraphim in the Temple, Is. 6), Yahweh was not to be represented by physical images. Scholars refer to this kind of worship as aniconic, meaning without image. Aniconic worship can take a variety of forms: it can exclude physical objects altogether or it can involve objects that have little to no representational quality (e.g. an unshaped meteorite which the god inhabits). Worship of rocks was common in the Near East, even into the Roman Empire.
All this is not to say that the Israelites were obedient to the Law of God. Physical representations of Yahweh have been discovered in Israel and elsewhere: e.g., a coin depicting Yahweh on his throne chariot (cf. Ezek. 1); a potsherd on which images of Yahweh and Asherah have been incised. These practice ceased (more or less) after the Exile, with the exception of the syncretism of the Hellenists during the time of Antiochus Epiphanes, when Yahweh was equated with Baal Shamin/Zeus Olympios.
Scholars have suggested a number of possibilities as to why the Israelites were forbidden to represent their God. One reason was the prevalence of sympathetic magic: practitioners of this art held that they could influence or control gods and spirits and other supernatural forces through the manipulation of physical objects, such as cult statues. For example, when a cult statue was made, a ritual known as the “opening of the mouth” (cf. the Egyptian ceremony performed on mummies) was performed, in which the spirit of the god was drawn into the statue. The statue and the god became identical on completion of this ceremony.
Worshippers of Yahweh were forbidden to engage in such practices because they were insulting to the Creator, and because they would lead to false ideas about Him, which in turn might lead to polytheism and syncretism. Yahweh is not reducible to the physical universe, and He cannot be manipulated: no one can overpower Him, and as the source of morality, He will not do what is immoral. God’s glory is beyond limit and comprehension. This theme was preserved through the Exile right into the New Testament period, as exemplified in verses such as the following.
And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things. Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
Another reason for banning images was to stress the monolatry, and later monotheism, of Israelite religion. This idea was embodied in the single cult site of Yahweh’s worship: Jerusalem. Following the completion of the Solomonic Temple, no other site in Israel was considered by the “orthodox” to be acceptable for sacrificial worship (though this did not stop disobedience). The one site reinforced the idea of the one God. Similarly, if images had been permitted, then there would have been no means to stop Israelites from making these images in their villages throughout the land. Multiple images would lead to multiple shrines, and this in turn would lead to polytheism and syncretism.
The Israelite worldview of the pre-Exilic period (and after in certain circles) was not strictly monotheistic in the modern sense of the world. They acknowledged the existence of other divine beings who were created by Yahweh. They were inferior to Him in all respects, but compared with humans, they were powerful and worthy of respect. The term “angel” (which is Greek for “messenger” and corresponds to Hebrew “malak”), originally referred to the lowest tier of these beings, their servants and messengers. As time went on, it came to be used as a general term for any spiritual entity that was not human and not Yahweh. Thus before this shift, the higher beings were referred to by other terms: Watchers, Gods, Sons of God, Hosts of Heaven, etc. This hierarchical structure was preserved in Jewish thought, but the language changed. Consider the following verse written by St Paul.
For by Him [Jesus] were all things created, that are in heaven, and that are in earth, visible and invisible, whether they be thrones, or dominions, or principalities, or powers: all things were created by Him, and for Him…
The features of “orthodox” Israelite religion were meant to preserve the notion that there were other spiritual beings (to whom the custody of the Gentiles had been entrusted, cf. Deut. 32:8, LXX, NOT MT), while simultaneously affirming that Yahweh was God of gods, with no rival.
Who is like unto thee, O Lord, among the gods? who is like thee, glorious in holiness, fearful in praises, doing wonders?