Christianity is a monotheistic religion, meaning we believe in only one God. Scripture is clear on teaching this important point of doctrine. It is also clear in proclaiming Who this one God is, the person of the Father of our Lord Jesus Christ:
“There is one body and one Spirit, just as also you were called in one hope of your calling; 5 one Lord, one faith, one baptism, 6 one God and Father of all who is over all and through all and in all.” (Ephesians 4:4-5 NAS)
“This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.” (John 17:3 NAS)
“yet for us there is but one God, the Father, from whom are all things and we exist for Him; and one Lord, Jesus Christ, by whom are all things, and we exist through Him.” (1 Corinthians 8:6 NAS)
Yet many Christians today want to ground their monotheism on the unity of the divine nature. They argue that although there are three persons of the Trinity, the Father, the Son, and the Holy Spirit, there is one God because there is only one divine nature shared by the three persons. This, however, is an unbiblical way to answer the question of why there is only one God.
Scripture never once grounds monotheism in a shared nature or essence considered in abstract; rather, the one God, as we have seen above, according to the scriptures is a person, the person of the Father. The Bible’s answer to why there is one God is to point to the person of the Father as that one God; on the other hand, many theologians prefer to skip this and ground their monotheism on the notion of common divine nature.
Ignoring the Bible’s explanation of monotheism cannot end well. Indeed, it has not. The argument that although there are three distinct persons, there is only one God because They all share one divine nature falls woefully short when held up to scrutiny. Three men, for example, all share one human nature; yet they are not one man because of their singular nature, but three men. We can say the same about horses, angels, and all sorts of creatures- a common nature shared by multiple persons does not prevent them from being plural horses or plural angels. So if our explanation of why the three persons of the Trinity do not constitute more than one God is that They have a common divine nature, we run into the obvious logical problem that on that basis it would still be legitimate to call Them three Gods.
The Bible, however, never grounds our belief in one God in a common divine nature or metaphysical essence, but always in the person of the Father. He is the one the Lord Jesus Christ called “the only true God”. He is the one Paul the apostle, under the inspiration of the Spirit, says is our “one God”. If we want to be monotheists in the way scripture teaches us to be, then our monotheism must be grounded in the person of the Father as the one God we believe in.
And in what way is the Father the one God? The Father is the one God, biblically, on account of two things that are peculiar to Him which the Son and Holy Spirit do not share: that He is the uncaused Cause of all, and that He is the Supreme Authority over all. His Son is also a cause of creation, as God created all things through Him, and likewise His Son has headship over creation. But even the Son and Holy Spirit are subject to the Father as Their Authority and Head, and are from Him by eternal generation and procession, as Their Cause. The Father alone is then the uncaused Cause of all, and thus is the one Supreme Authority over all. Therefore, as there is only one uncaused Cause of all, and one Supreme Authority, there is only one God, the Father Almighty.