When we come to scripture, and examine God and the Trinity, we are faced with a few clear facts revealed in scripture:
1) There is only one God (1 Tim 2:5, James 2:19, Mark 12:32).
2) There are three divine persons, the Father, Son, and Holy Spirit (John 3:16, John 1:1, Acts 5:4, Rom 8:14, Matt 28:19, Matt 3:16-17).
3) The one God is one of these persons, the Father (1 Cor 8:6, John 17:3, Eph 4:4-5).
Examining these facts, we may be inclined to ask why, biblically, do the Son and Holy Spirit do not constitute a second and third God?
When we begin to answer this question, we can’t point to the Father’s divinity as the reason that He is the one God, or that the Son and Spirt aren’t second and third Gods, since the divinity of all three persons is exactly the same. The Father’s divine nature is identical to that of the Son and Spirit. Therefore, we can’t look to some difference in the divinity of the persons to explain this.
What we can do is ask, ‘what makes the Father unique, such that scripture would call Him in particular the one God?’, and extrapolate from that by good and necessary consequence what the factors are that make the Father alone the one God, which the Son and Spirit don’t have. And when we examine that we can come up with two things, namely, that the Father alone is uncaused, and that He has no higher authority than Himself. He alone is the uncaused cause and head without a head.
We can then look at these factors and see that there is really something distinct about the Father, besides simply that He is Father, for which reason He is regarded as the one God, qualities which the Son and Spirit don’t share with Him, and thus They don’t constitute another God or Gods.