Short answer up front: You can’t “disprove” a verse theologically for believers, but from a historical-critical viewpoint there is strong, widely-accepted evidence that Isaiah 44:6 (and similar late prophetic proclamations of absolute, sole divinity) reflect a later monotheistic theology and do not describe how the earliest Israelites actually conceived the divine realm. Archaeological and textual evidence from Canaan and Ugarit, plus internal biblical traces (e.g., Deut 32; Ps 82), show an earlier West-Semitic religious landscape in which “El / Elyon” and a divine council existed and where Yahweh appears in texts as connected with—sometimes distinguished from—El/Elyon. Below I give the main lines of evidence, expert quotations, and reliable sources so you can judge for yourself.
1) The relevant claim in Isaiah 44:6 and what it does
Isa 44:6: “I am the first and I am the last; and there is no god besides
me.”
Scholars read this as late, polemical monotheistic language—a
theological assertion aimed at affirming Yahweh’s exclusive status after
centuries of religious struggle and exile, not necessarily a report about
earlier Israelite religious practice. OUP
Academic+1
Expert paraphrase: “Isaiah 40–55 bears witness to an incipient/explicit monotheism developed in late exilic/post-exilic contexts.” Brill
2) Ugaritic/Canaanite evidence: El, Elyon, a pantheon and the divine council
Texts from 
Quote (summary of scholarly consensus): “The Ugaritic material shows El as the head of a pantheon; many Old Testament divine-council motifs fit this background.” Quartz Hill School of Theology
3) Biblical traces that preserve an earlier, non-exclusive divine worldview
A number of biblical passages preserve older material that scholars read as vestiges of henotheism/functional polytheism:
·        
Deuteronomy 32:8–9 (Dead Sea /
Septuagint variants): early readings imply Elyon “divided the
nations” among divine sons, and Yahweh received 
·        
Psalm 82: a scene of the
“divine council” where God judges the gods—this implies a belief in other
divine beings judged by the Most High. Scholars argue this is a holdover of
earlier divine-council imagery in 
Michael Heiser (summarizing the evidence): “Scripture contains vestiges of a divine council worldview in which Yahweh operates amid other heavenly beings.” Scholars Crossing
4) How El / Elyon and Yahweh were related (two scholarly models)
Scholars offer two common reconstructions:
1.      Identification/Merge
model — over time the attributes and titles of El/Elyon were absorbed
into Yahweh, so Elic titles (El, Elyon, Shaddai) become names/titles
of Yahweh in 
2. Henotheistic origins model — originally a pantheon: El (Elyon) head, with sons (national gods) including a god called Yahweh who eventually becomes the national god of Israel; later religious reformers and prophets fused or subordinated the others to Yahweh and promoted exclusive monotheism. Deut 32 and Psalm 82 are often read in this light. Scholars Crossing+1
Either way, the historical point is the same: Isaiah 44:6’s absolute claim is a later theological development, not a mirror of the earliest Israelite religious imagination. Brill
5) Selected expert quotations (short, representative)
· Mark S. Smith (leading Ugaritic/Israelite religion scholar):
“The biblical deity of 
· From a Cambridge/Oxford survey of the rise of monotheism:
“Verses declaring ‘YHWH is one’ become the scriptural foundation on which later thinkers base a theology of God’s unity and transcendence.” (i.e., the unity language is a theological development). Cambridge University Press & Assessment
· On Deut 32 / divine-council language:
“At various sites Yahweh was incorporated into the older figure of El … the biblical record preserves traces of this merging.” OUP Academic
· On Isaiah 40–55 specifically:
“Isaiah 40–55 is often understood as a work bearing witness clearly and unambiguously to an incipient monotheism” (showing it is theological proclamation in its context). Brill
6) What this means for the claim that Isaiah 44:6 is historically false
· Historically (descriptive claim about ancient Israelite beliefs): The weight of textual and archaeological evidence shows that earlier Israelite religion contained ideas and language consistent with a pantheon / divine council, and that Yahweh’s sole-divine status was not uncontested or fully formed in the earliest layers. In that sense, Isaiah 44:6 does not describe the early religion—it reflects a later doctrinal claim that developed after these older ideas. Quartz Hill School of Theology+1
· Theologically (normative claim about God’s nature): Isaiah 44:6 remains a religious/theological assertion. Historical evidence can show it was formulated in a particular historical/theological context, but whether the verse is “true” in a faith sense is a separate question outside historical method. OUP Academic
7) Key sources you can read next (accessible, scholarly)
·        
Mark S. Smith, The Early
History of God (Harper & Row / Princeton 1990/2002) — foundational on
El, 
· Michael S. Heiser, The Unseen Realm (popular but referenced academic work) and his articles on Yahweh & El (useful introductions to the divine-council literature). Scholars Crossing
·        
Seth Schwartz, Imperialism
and Jewish Society (
·        
Brill / 
Short conclusion
If your goal is to show that Isaiah 44:6 does not reflect earlier Israelite polytheistic contexts, the evidence and mainstream scholarship support that claim strongly: Isaiah 44:6 is best read as a later, polemical proclamation of exclusive monotheism, composed in a milieu that was replying to older expressions of a divine council and many-gods background. If your goal is to “disprove” the verse for readers of faith, historical criticism can undermine the claim that this language describes the earliest religion, but it cannot by itself settle theological truth claims.
 
