[Zancarius]

Trinitarianism isn't necessarily a strictly Christian construct—or rather the idea of a godhead comprising multiple parts. "Two Powers" theology (a transcendent, unseeable Yahweh; and Yahweh-as-man) was accepted by Jewish thinkers until about the First Century AD, largely due to Christian influences. It's visible in passages like Genesis 19:24 (two Yahwehs) and most "angel of the Lord" language (e.g. Judges 6:11ff).

Alan Segal's Two Powers in Heaven delves into this in great detail.

[andsoitis]

From https://plato.stanford.edu/entries/trinity/trinity-history.h...:

Divine threesomes abound in the religious writings and art of ancient Europe, Egypt, the near east, and Asia. These include various threesomes of male deities, of female deities, of Father-Mother-Son groups, or of one body with three heads, or three faces on one head (Griffiths 1996). However, similarity alone doesn’t prove Christian copying or even indirect influence, and many of these examples are, because of their time and place, unlikely to have influenced the development of the Christian doctrine of the Trinity.

A direct influence on second century Christian theology is the Jewish philosopher and theologian Philo of Alexandria (a.k.a. Philo Judaeus) (ca. 20 BCE–ca. 50 CE), the product of Alexandrian Middle Platonism (with elements of Stoicism and Pythagoreanism). Inspired by the Timaeus of Plato, Philo read the Jewish Bible as teaching that God created the cosmos by his Word (logos), the first-born son of God. Alternately, or via further emanation from this Word, God creates by means of his creative power and his royal power, conceived of both as his powers, and yet as agents distinct from him, giving him, as it were, metaphysical distance from the material world (Philo Works; Dillon 1996, 139–83; Morgan 1853, 63–148; Norton 1859, 332–74; Wolfson 1973, 60–97).

Another influence may have been the Neopythagorean Middle Platonist Numenius (fl. 150), who posited a triad of gods, calling them, alternately, “Father, creator and creature; fore-father, offspring and descendant; and Father, maker and made” (Guthrie 1917, 125), or on one ancient report, Grandfather, Father, and Son (Dillon 1996, 367). Moderatus taught a similar triad somewhat earlier (Stead 1985, 583).

[Zancarius]

I think I tend to agree with the conclusion—namely that it was predominantly a Jewish influence on Christology. Part of my opinion is shaped by changes in and around the First Century in Judaism which, ultimately, culminated in the Masoretic Text evicting certain parts of the text that could remotely suggest anything akin to polytheism. Deuteronomy 32 is particularly one of the most affected chapters, but curiously "two powers" theology was largely left intact.

This is a particularly interesting period in Christianity, because you had numerous influences (including what would later become gnosticism around the same time), the term "trinity" wouldn't appear in extant works until sometime in the Second Century, then the Council of Nicaea in or around the latter half of the Fourth Century establishing it as doctrine.

So, I think the evidence of a decidedly Jewish influence is quite strong and would date at least to the Babylonian captivity.

I believe Dr. Robert Alter leans toward an evolution from polytheism -> monotheism in Jewish thinking (I highly recommend his The Hebrew Bible: A Translation with Commentary); but I think the evidence for a sort of henotheism is a bit stronger and more sensible, which would better fit the sources you shared here, alongside the biblical texts.