This is a common misrepresentation. Christianity is as firmly monotheistic as Judaism. What Christians believe is that this one God exists in three persons or personalities. This belief is based on the Scriptures, both Old and New Testaments.
We affirm that the Hebrew Bible teaches the oneness of God. The cardinal affirmation of the Jewish people has always been the Shema: ‘Hear O Israel: the Lord our God, the Lord is one.’ Jesus also affirmed the Shema. (Mark 12:29)
The Shema uses the Hebrew word, echad. This word allows for a plurality or diversity within unity. Indeed the common Hebrew word for God, Elohim, is also plural in form.
There is a word in Hebrew to describe an indivisible unity, yachid. Maimonides, when composing his famous Thirteen Articles of Faith, substituted yachid for echad in describing the nature of God. Ever since, the notion of an indivisible unity of God has been fostered in Judaism. However, the Bible gives ample instances to show that there is diversity within God’s unity.
The Zohar, the foundation book of Jewish mysticism, also recognized that the idea of a plurality-in-unity is not foreign to Jewish thinking.
In the Hebrew Scriptures there are two other personalities who are portrayed as distinct from, yet somehow the same as, God. These are the angel of the Lord and the Spirit of God or the Holy Spirit.
The angel of the Lord is sometimes identified with God himself e.g. Genesis 16:7, 13; 22:11-12.
God’s Spirit is spoken of in Scripture as a personality of his own, yet identified as God, e.g. Genesis 1:2, Psalm 51:11, Isaiah 11:2.
In the New Testament, the three personalities mentioned above are portrayed as God the Father, God the Son and God the Spirit, without compromising the fundamental affirmation of the Shema.
People often say, “Don’t Christians believe that Jesus is the Son of God and also God. And therefore they are making a man into God, and anyway God doesn’t have a son.”
In 1 Chronicles 17:13, the King of Israel is referred to as God’s ‘son’. The Talmud also states that the Messiah would be God’s son. The idea in the Scriptures is not that a man became God, but that the Messiah would himself be God coming as a man. Isaiah 9:6 portrays the coming of the Messiah in this way, ‘For to us a child is born, to us a son is given and the government will be on his shoulders. And he will be called Wonderful Counselor, Mighty God, Everlasting Father, Prince of Peace.’ But if God is indeed a ‘tri-unity’, then it is possible for the Messiah both to be called God and to exist in a relationship characterized as ‘Son of God’.
Why do Christians worship three Gods?