A basic tenet of Christianity is monotheism, a doctrinal issue which it shares with both Judaism and Islam. However, within Christian monotheism we have a number of theological aspects which challenge the concept of only one God. Foremost among these is the doctrine of the Trinity, which subdivides God into the Father, Son, and Holy Spirit. Both Orthodox Judaism and Islam see Christianity's Trinity as a denial of monotheism. The following quote is an example of the Judaic position on this subject :
While Judaism believes that G-d manifests Himself to His creation (humanity) in many ways, (i.e. as a judge or a protector) G-d’s essence itself is indivisible and therefore without any possibility of distinction. Something that transcends both time and space cannot be described as consisting of three different aspects. The moment we attribute any such distinctions to G-d’s essence, we negate His absolute Oneness and unity.
Whereas Christianity assumes that one God can exist as three distinct parts within the span of human history in order to bring about God's purpose, Judaism holds that monotheism means not just that there is only one God but that God is also indivisible and immutable, outside of time and therefore unchangeable and incapable of subdividing).
Thus we see that monotheism as a religious attribute is far from fixed. Even this most basic doctrinal point is shrouded in complexity and disagreement.
Trinitarian doctrine has another aspect which raises serious a question. Does or did the Trinity have a pecking order. Which aspect or subdivision of the Godhead is supreme or are all aspects equal. Between the Father and the Son, that question is quite confused. In some places the NT seems to indicate that Jesus and God the Father are equal. Then in other places Jesus is said to be subordinate. If all aspects of the one God are not equal in power and authority, is the unity and oneness of monotheism violated? This question goes back to one's basic definition of monotheism.
As far as Christian monotheism is concerned, we also must note that within the Old Testament Scriptures, which form the basis for both Christianity and Judaism, God is called by a variety of names in the Hebrew- El Shaddai, Yahweh, Elohim, I am, Jehovah, etc. Multiple names suggests the possibility of multiple gods. Islam recognizes as much in its insistence that the one God is called only Allah.
Within the Christian New Testament, God is most clearly depicted as Trinitarian, raising the associated issues relative to monotheism. In addition the NT also uses multiple names to designate God: Father, Lord, Love, Alpha and Omega. Again this multiplicity of designations offers the chance for confusion about whether the same deity is always under consideration.
Finally and perhaps most significantly, within both the Old and New Testament scriptures we encounter various ways in which God is depicted in terms of temperament and attitude toward humanity. God is alternately loving, angry, jealous, protective, threatening, constant, vacillating, patient, frustrated, rigid, mysterious, and caring. Whatever description or concept of the divine one might want, there is a portion of the Bible to support that particular God concept. If one God can have so many different personalities, that one God is little different from the pagan pantheon of gods. God of War. God of Peace. God of Love. God of Hate. They are all there for the picking.