The one and only true God exists in three persons: Father, Son, and Holy Spirit. The Father is God. The Son is God. The Holy Spirit is God. And there is only one God. All three persons of the Trinity are eternal – they were not created, and there has never been, nor will there ever be, a time when they did not all exist. All three persons are active at all times and in every moment. Father, Son, and Holy Spirit are all divine. God is relationship: the Son is begotten of the Father, and the Spirit proceeds from or is breathed by the Father. Everything that is true of the Father is also true of the Son and of the Holy Spirit. They, together in three persons, are God.
Sometimes, in order to understand what is, it is important to understand what is not. For example, adoptionism is a heresy that claims that Jesus is less than the Father and is an ordinary human who, through his moral progress, merited adoption by God. Despite what the believers of adoptionism think, Jesus did not earn his divinity by being morally good. Jesus is and always has been divine and is not less than God. He is God.
Modalism is another heresy, and it paints the Trinity as three modes or roles in which God works rather than them being the truth about who the one God is. Father, Son, and Holy Spirit are not roles or modes; they are God.
Arianism is a third heresy. In this belief, Jesus is like God in almost every way; however, he is seen as a created being – God’s first and best creation – rather than being eternal and uncreated. Believers of Arianism see Jesus and the Holy Spirit as subordinate to God. In reality, Jesus and the Holy Spirit had no beginning. They are eternal and divine, as only God can be. Therefore, Jesus and the Holy Spirit are God.