The conception of God held by the Rabbis is monotheistic in the strictest degree.

He created in the beginning one man only, so that heretics should not say that there are several Powers in heaven (Sanh. 38a).

On the verse “Hear, O Israel, the Lord our God, the Lord is one”  the comment is made: “The Holy One, blessed be He, said to Israel, ‘My children, everything that I created in the Universe in is pairs – e.g. heaven and earth, the sun and moon, Adam and Eve, this world and the World to Come; but I am one and alone in the Universe’ (Deut. R. II. 31).

The Rabbi’s define the Christian Trinity as three gods in one God, as can be seen in their attempt to answer the Christian doctrine of God.

What of that which is written, ‘Let us make man in our image, after our likeness’ (Gen 1:26)? … Read what follows: it is not said, ‘And gods created man in their image,’ but ‘And God created man in His own image. … In the past Adam was created from the dust of the ground and Eve was created from Adam.  Henceforward it is to be ‘in our image, after our likeness’ – meaning, man will not be able to come into existence without woman, not woman without man, not both without the Sheckinah.’ (R. Simlai asked by the Minim) (emphasis original).[i]

The Sheckinah explains ‘our’ to mean God in addition to man and women; i.e. each human being is formed from three parents; a threefold parentage (Nid. 31a).  The threefold parentage being man, woman and God.  This is the Jewish explanation to the plural noun usage.