I'm going to rearrange the question to answer it more pointedly to deal with the exegesis first; the quotations are all answered here but in another order; also the caps are adding emphasis, please don't edit.
"I am under the impression that the Trinity is a Christian idea", and that the Jews did not view God as "three in one and one in three".
In this portion of the question it is assuming that the "Trinity", or "three in one and one in three". is implied by whom?
- The speaker
- The writer
- The translator
- The reader
In this case it is by the reader importing his knowledge into the question this is absolutely the case as the word Trinity and the phrase "three in one and one in three" does not appear in Genesis thus far. The question whould be perhaps better asked plainly in the latter part alone, and then see if the text offers from it's own exegesis as to how this could be understood:
How, then, was the following passage interpreted by the people of God prior to Christianity arriving on the scene?
Then God said, “Let us make mankind in our image, in our likeness -Genesis 1:26
- ויאמר אלהים נעשׂה אדם בצלמנו כדמותנו וירדו בדגת הים
There are 4 parts to this statement that should draw a hermenutic students attention:
Because in Hebrew the Noun for God- אלהים is a singular plural, not much concerning speaker אלהים - God(s) can be attained but that deity in the Hebrew sense offers plurality of speech even from one speaker if the speaker is deity, yet אלהים did not 'speak' to one and other in the plural, but rather אלהים "said" , thus implying one speaker, making a statement.
The following points are also of interest as to אלהים nature of this proclaiming to make Adam/man upon the earth:
The Verb: נעשׂה -let us Make
A Noun: בצלמנו - in our Image
A Noun: כדמותנו - in our Likeness
There are 3 plural distinctions and pluralities to be noticed as made by the 1 אלהים- God,
1. the Making of man by US,
2. That man will be in OUR image,
3. That man will be in OUR likeness.
Is this trifold statement of creating of man significant with a trip fold nature, that of being Made, being made with an Image, being made with an image and Likeness?
This depends on what is revealed concerning the
אלהים -God prior in the text of Genesis 1 to find if this Making of man, that will be After God - אלהים in Image and Likeness, what is revealed of his nature or being.
And in the first revealation of God to us from the Text is as well seen אלהים 's nature speaking in the singular command, making God (singular) an accurate choice to discribe אלהים :
Genesis 1
1 In the beginning God created the heaven and the earth.
2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
3 And God said, Let there be light: and there was light.
From this first revealation we have to comport into the Gen 1:26 question is how God interfaces with creation, which can be seen in:
- God created
- the Spirit of God moved
- And God said
While further revealation would show how the God head used his various functionaries, this is more than enough evidence to suggest that God interfaces in creation and imparts to man a trifold nature after his own, in:
1: God the Creator having being and thus making man to have being,
2. God having an Image in Spirit and making man to have his Image in Spirit,
3. God having Likeness in what is "Said" (logical word - logos) and making man to have Likeness after God's "said" ability.
The "said" Likeness is the hardest to communicate with out using a new tense of speaking, but it would best be said "His Word" Hebrew: DeBaR, Greek: Logos.
So while Christians are often taught trinitarian theology in context of total revealation of scripture, the OP's assertion that:
"I am under the impression that the Trinity is a Christian idea", and that the Jews did not view God as "three in one and one in three".
On accurate, but it is not that a trifold nature could not be known of the one God אלהים.