Question: "What does it mean that the LORD is one (Deuteronomy 6:4)?"
Answer: The opening of the Shema (or the “Saying”), a central teaching in Judaism, says that the Lord is one: “Hear, O Israel: The LORD our God, the LORD is one” (Deuteronomy 6:4).
Most English Bibles include a footnote to express alternative translations, as this is a difficult passage among Hebrew scholars. Options include “The LORD our God is one Lord,” “The LORD is our God, the LORD is one,” and “The LORD is our God, the LORD alone.” In all the options, the focus is on the idea of one God. The doctrine of one God was a stark contrast to the theologies of the cultures surrounding the Israelites. Other religious systems, including that of the Egyptians, served a wide variety of gods and goddesses. The worship of only one God made the faith of the Hebrews unique in the ancient world.
Exodus 20 gives the Ten Commandments. It also begins with an emphasis of God as one: “You shall have no other gods before me” (Exodus 20:3). God revealed Himself as the one God to worship. There could be no other.
The origin of monotheism was not Deuteronomy 6:4, however. The opening words of the Bible are “In the beginning, God created the heavens and the earth” (Genesis 1:1). Only one God was before all things and created all things. This same one God was the One who spoke with Adam and Eve in the Garden of Eden (Genesis 2—3), saved the world through Noah (Genesis 6—8), and promised Abraham that all nations would be blessed through him (Genesis 12). Israel was always taught that the Lord God was the one God; the Jews were to reject all idols and deny all other gods.
If the Lord is one, how are we to understand the Trinity? Though the word Trinity is not found in the Bible, the concept certainly is. The Father, Jesus Christ, and the Holy Spirit are each referred to as God and are attributed qualities that only God has. For example, Jesus was in the beginning with God (John 1:1), and all creation was made through Him (Colossians 1:16–17). The Holy Spirit is listed with Father and Son as “the name” believers are to be baptized into (Matthew 28:19–20) and was referred to as God by Peter in Acts 5:3–4. The teaching of a Triune God is unique to Christianity and affirms Father, Son, and Holy Spirit as one God in three Persons.
Christians believe that God is one yet is also triune. We know God through faith in Jesus Christ (John 14:6), and the Holy Spirit works within us to help us live for God each day.