As the Scriptures clearly teach, there is only one God. Judaism, Christianity and Islam are called Monotheistic for this reason. However, a strict and bare monotheism leaves us with some problems. First, How do we deal with Scripture passages such as these:
"The LORD appeared to Abraham ... Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance to his tent to meet them and bowed low to the ground. He said, 'If I have found favor in your eyes, my Lord, do not pass your servant by." -- Genesis 18:1-3 NIV.
"Come near me and listen to this: From the first announcement I have not spoken in secret; at the time it happens, I am there. And now the Sovereign Lord has sent me, with his Spirit." -- Isaiah 48:17 NIV. (Who is speaking here?)
"The Lord says to my Lord: "Sit at my right hand until I make your enemies a footstool for your feet." -- Psalm 110:1 NIV. (A psalm of David. Who is speaking here? Is it the Messiah, the son of David? If so, how can David (speaking by the Spirit) call him his 'Lord'? If he is his 'Lord', how can he be his son?)
"Hear O Israel: The Lord our God, the Lord is one." -- Deut. 6:4 NIV.
Even in this last passage, the great Shema, the closest thing to a creed in Judaism, which strongly affirms the oneness of God, there are hints of complexity. The word used for one is achad, which means a unified one, not rashid, which means a single isolated one. And even the great Jewish scholar Moses Maimonides pointed out the interesting fact that the name of the Lord is called three times in the Shema. Why is this?
A second problem that arises in strict Monotheism is the philosophical question of the origin of diversity. If the Ultimate Principle is one thing and has no complexity or diversity, then how can anything NOT be God? This leads directly to Pantheism, which the Scriptures consistently reject, or Manicheism, an early heresy that posited two ultimate and opposite principles (Dualism). This view creates other problems, such as the loss of God's sovereignty. What is the solution?
The Trinity doctrine claims that the Ultimate Principle contains an aspect of diversity as well as of unity.