First, monotheism (“mono” = “one” and “theos” = “God”) or the reality of God being One God in regards to His Divine essence or nature (Deuteronomy 6:4-5; Matthew 22:37). God’s oneness of being is without dispute in both Old and New Testaments (Genesis 1:1; 1 Corinthians 8:6). The “Oneness” of God in the Bible is not just a bare oneness, but evidently is hinted at in the Old Testament as God being One God that has the capacity to relate within Himself as a “compound unity” (see Genesis 1:26 i.e “make man in “our image”). This One God revealed Himself not only as the Creator (Genesis 1:1; John 1:1) but also Personally in regards to His redemptive dealings as the Father (Deuteronomy 32:8; Hosea 11:1; John 17:5).
The second fundamental truth revealed about God in the Bible has to do with the Deity of the Son – the Lord Jesus Christ. As we noted earlier, the Old Testament hints that God is a compound unity”. He is identified as “Father” nearly ten times in the Old Testament Who has with Him a Second Personage – the Son (Psalm 2; Proverbs 30:4). The New Testament fully reveals this Personage to be none other than “The Son” who came as the fullness of God in human flesh (John 1:14; Colossians 2:9). Jesus Christ repeatedly asserts His Deity (John 8:58); eternal pre-existence (John 17:4-5) and other Divine qualities (Matthew 28:18) that He retained but chose to not access for the sake of His assumption of true humanity (Philippians 2:5-11) in His sojourn on this earth. The New Testament equally asserts both Old Testament Monotheism and Christ’s Deity as comprising the singular identity of the One God (1 Corinthians 8:6-7).
Then the third fundamental truth that describes this idea of God being “one God that exists eternally in three persons” is that truth of the Personality of the Holy Spirit. In so far as Jesus Christ is equal in all respects to the Father in their sharing of full Deity, the Holy Spirit is described by Jesus as being another Person of like kind to Himself and the Father (John 14:16; 15:26; 16:8-15).
By understanding God as monotheistic in regards to His essence, the Deity of the Son in His relationship to the Father and the Personality of the Spirit in His relationship to the Father and Son, we then come lastly to the fourth truth comprising the doctrine of the Trinity– God as a Personal God. Over time many people have sought illustrations from the physical world to illustrate this profound truth of God being “One” and yet “Three” at the same time. All analogies eventually break down despite their initial aid in helping people to approach this truth.