Trinity

There are three persons in the Godhead; the Father, the Son, and the Holy Spirit; and these three are one God, the same in substance, equal in power and glory. (1 John 5:7, Matt. 28:19)

Further Study: God has revealed himself as a trinity (Three yet One).
God is not three people - just one, but God also reveals himself as God the Father, God the Son (Jesus) and God the Holy Spirit. The Spirit is neither the Son nor the Father. The Son is neither the Father nor the Spirit and the Father is not the Spirit nor the Son - they are distinct, but not separate as three Gods - God is one. It is a very difficult doctrine to understand - indeed no one can fully understand it, but that is how God has revealed himself to us.
An excellent addition to comprehending the Trinity are the several creeds of Church history - including the Nicene Creed and in particular the Athanasian Creed. Additionally take careful note of the two charts here as they assist you in your study.

Consider also these related posts:

• There is Only One God | Truth is Still Truth
• The Supremacy of God the Father | Truth is Still Truth
• "The Trinity is Like 3-in-1 Shampoo". . . And Other Stupid Statements | Parchment and Pen

The Trinity is one of the most boggling concepts in the scripture, and all of our struggles to comprehend it fall short.

One of the most complete statements on the trinity is the Athanasian creed. Written perhaps around 500 AD (up to two hundred years after Athanasius (died Ca 373AD))
¹ It focuses on explaining the trinity as a doctrine central to the Christian faith, even adding that it is necessary for salvation to believe in the Trinity.

Whosoever will be saved, before all things it is necessary that he hold the catholic faith; Which faith except every one do keep whole and undefiled, without doubt he shall perish everlastingly.
And the catholic faith is this: That we worship one God in Trinity, and Trinity in Unity; Neither confounding the persons nor dividing the substance. For there is one person of the Father, another of the Son, and another of the Holy Spirit.

‎There is only One living and true God. (Deut. 6:4, Jer. 10:10)

Further Study:
• While people all over the world may believe that there are many gods, and may worship many gods, there is only one living and true God.
• God by definition is the greatest of all, there can only be one who is greatest of all - so there can only be one God.