How is it possible to believe in the Old Testament in which there is just one God and at the same time believe in Trinity?

Believing in the Old Testament's monotheistic view of God and simultaneously embracing the doctrine of the Trinity involves understanding the nuances of both perspectives. Here's how these beliefs can coexist:

Understanding Monotheism in the Old Testament

The Old Testament emphasizes the unity of God, as seen in passages like Deuteronomy 6:4, which states, "Hear, O Israel: The LORD our God is one LORD." This verse underscores the monotheistic nature of Jewish belief, affirming that there is only one God35.

The Concept of the Trinity

The Trinity posits that there is one God who exists as three distinct Persons: the Father, the Son, and the Holy Spirit. Each Person is fully God, yet they are not separate gods but rather one God in essence23. This doctrine affirms monotheism by asserting that there is only one divine essence or being23.

Reconciling Monotheism with the Trinity

1. Plural References in the Old Testament: Some passages in the Old Testament hint at a plurality within God, such as Genesis 1:26, where God says, "Let Us make man in Our image." This use of the plural can be interpreted as a foreshadowing of the Trinitarian concept5.

2. Hebrew Understanding of Unity: The Hebrew word "echad" (used in Genesis 2:24) signifies a united oneness, as opposed to "yachid," which means a solitary oneness. This distinction supports the idea that God's unity can encompass multiple Persons1.

3. Essence vs. Person: The Trinity distinguishes between God's essence (what He is) and His Persons (how He relates). All three Persons share the same essence, making them one God, not three separate gods23.

4. New Testament Clarification: The New Testament provides clearer teachings on the Trinity, showing how Jesus and the Holy Spirit are distinct yet fully divine, alongside the Father24.

In summary, believing in both the Old Testament's monotheism and the Trinity involves recognizing the Old Testament's hints at divine plurality and understanding the Trinitarian concept as affirming one God in three distinct, fully divine Persons.