There is one required assumption in order to interpret correctly these passages: that John holds these passages as divinely inspired and therefore true:
"You are my witnesses," declares the LORD, "and my servant whom I have
chosen, so that you may know and believe Me and understand that I am
He. Before Me there was no God [el] formed, neither shall there be after Me. (Is 43:10)
"Thus says YHWH, the King of Israel and his Redeemer, YHWH of hosts:
'I am the first and I am the last, and there is no God besides Me. (Is
Do not tremble and do not be afraid; have I not told you from of old
and declared it? And you are my witnesses! Is there a God [eloah] besides Me?
There is no Rock; I know not any.'" (Is 44:8)
"I am YHWH, and there is no other; besides Me there is no God. I will
gird you, though you have not known Me; (Is 45:5)
"Declare and set forth your case; indeed, let them consult together.
Who has announced this from of old? Who has long since declared it? Is
it not I, YHWH? And there is no other God besides Me, a righteous God [el]
and a savior; there is none except Me. (Is 45:21)
First, it is well-known that the name Elohim, literally "the gods", has two meanings in the Hebrew Bible: a) the only omnipotent, eternal Creator God, YHWH, in which case the plural has a majestatic sense and the accompanying verb is singular, and b) the gods, either existing super-human entities created by and subordinated to YHWH (in pre-exilic texts only, of which I make the case below) or the imaginary gods of the gentiles, in which case the accompanying verb is plural.
Thus, verses like 44:6 and 45:5 which say literally "besides Me no Elohim/elohim" can be understood in either of two senses, depending on the sense of Elohim/elohim: with "Elohim" in sense a, as "besides me no omnipotent, eternal Creator God", and with "elohim" in sense b, as "besides me no gods", so that someone intent on affirming that post-exilic Jews did NOT reserve the term "god" for YWHW can reject these verses as proof to the contrary by saying that they use Elohim in sense a only, and therefore do not preclude the existence of lesser, subordinated, created gods which should not be worshipped.
Therefore I emphasized above the instances where "God" translated a singular Hebrew name, either "el" (Is 43:10) or "eloah" (Is 44:8), which I gave between . From these instances, it is clear that for post-exilic Jews all names referring to a divinity, both the majestatic plural "Elohim", which in Greek would be "ho Theos", and the singular "El" and "Eloah", which in Greek would be "Theos", were reserved to YHWH. The other existing superhuman real entities, the angels, were created by YHWH and wholly subordinated to Him, so that they could not be called "gods". And the "gods" of the idolatrous peoples were not real: "For all the gods [elohe] of the peoples are idols, but YHWH made the heavens" (Ps 96:5).
I wrote all the above to preemptively dispel the notion that the Apostle John could be referring by "theos" to a created superhuman entity, as non-trinitarians posit.
Turning to the NT, the term "ho Theos" or its genitive "tou Theou", dative "to Theo", or accusative "ton Theon", refer to God the Father, except in the 5 passages where it refers to the Son, none of which calls Jesus simply "ho Theos" without qualification: Mt 1:23, Jn 20:28, Ti 2:13, 2 Pe 1:1, 1 Jn 5:20. But even in these cases "ho Theos" refers always to a divine Person, not to the divine ousia.
On the other hand, unarthrous "Theos" can refer to:
the one and only divine ousia, what each divine Person Is, in which case it is the attribute of a copulative sentence whose subject is the Son (Jn 1:1, Rom 9:5) or the subject of a passive predicative sentence;
a divine Person, usually God the Father when it appears without qualification or the Son in "monogenēs Theos" (Jn 1:18).
So, Jn 1:1 says:
"In the beginning was the Word, and the Word was with God the Father, and the Word was all that God the Father was (except Father)."
Where from monotheism, "all that God the Father was" is understood in a sense of numerical identity, not of merely qualitative identity. "I and the Father are one" (Jn 10:30), not "I and the Father are equal". "Homoousios", i.e. "of the same ousia" (numerical identity), not "isoousios", i.e. "of identical ousiai" (merely qualitative identity).