Both Trinitarians and Oneness believe there is one God. But it is how we define it that makes the difference.
The Oneness teaching takes all the Scriptures that say God is one and purposely ignore the statements of Jesus and the gospel writers that speak of two or three persons of the Godhead. In the Old Testament teachings were focused on God being one in contrast to the polytheism of the nations that surrounded Israel. There are statements that speak of the plurality of this ONE God in the Old Testament record. However, it is only when we come to the New Testament that this one in unity is explained.
Bernard writes that the word apparently can mean both one in unity and one numerically for Strong's defines it as "united, one, first." He then gives just a few examples to prove that we should choose its meaning to be an absolute one.(Oneness of God, pp.152-153) Bernard finds a few examples contrary to the plural meaning which in no way is exhaustive. However the context determines the meaning.
In the Old Testament God is described as one. Duet.6:4 says, "Hear, O Israel: LORD our God is one: what Oneness does is camp on this passage, milking it till the cows come home. The phrase "one Lord" is preceded in the Hebrew by elohenu; it is our God is one. The word for one is not a numerical one but is actually a united one. The Hebrew word for one is echad, which comes from the root word achad, which means to unify or collect together (the intensive reflexive form signifying to unite). If this was meant to be a strict numerical statement the Holy Spirit would have had Moses use the word yachid, which means a absolute one, single, only one. Yachid is used twelve times in the Scriptures NOT ONCE is it used for Jehovah God. Despite all the explanations we can see they fail in light of the consistent usage of the word one throughout.
The Bible defines how the word one is used.
Gen.1:5 evening and morning are called one day ( a combination of two parts to make one) they are both considered a day yet we can distinguish them as different phases.
Gen. 2:24 Adam and Eve become one flesh (Here two personalities who come together in marriage and are one, not one person but in unity. God sees them as one even though they are not physically fused together like Siamese twins. If we take the Oneness view this would be the only consideration for our understanding.
Gen.11:6 the people are one Ezra 2:64 the whole assembly of Israel is like one.
Num. 13:23 according to their view When the spies went over into the land of Canaan they brought back one grape (Heb. eschal echad.) Thats one big grap ! Can anyone actually think it was a numerical statement. It means a cluster of grapes.
Ps.133:1 the brethren is to dwell as one ( in unity) 1 Sam.3:17 they are called one company 2 Sam. 2:25 one troop 1 kings 7:42 one tribe 1 Kings 11:13 Israel is called one nation.
Ez. 37:17 Ezekial is told to put two sticks together and combined they become one stick. Showing the nation would be unified. In all these examples can anyone find them to be a strict singular meaning ? This same word is applied to the one God and is clearly used as a compound unity. You can twist and turn at the truth of the matter , you can be uncomfortable in its teaching but you cant remove its consistent usage in the scripture.
The word for a strict single is yachid it is used in Gen.22:2 "Take thy one and only son." This can also be used for Gods only son being unique and one of a kind.
PLURAL STATEMENTS OF GOD IN THE N.T
Vs.20 "I am in my Father and you are in me and I in you in vs.23 he speaks in a plural ... and we will come to him and make our home with him. Is the humanity of Christ going to make its home in us! This is one of the few times Jesus speaks in a plural context to explain his unity with both the Father and the Holy Spirit.
Essentially the word one is describing the being of God while we have other statements that describe the persons of God (not human persons but distinct identities). There is not conflict unless someone disregards one or the other.
Jn.17:21 Jesus prays that they all may be one, as you Father are in me , and I in you , that they also may be one in us."
In the New Testament there is a Greek equivalent to the Hebrew word for one. In Mt.19:5 Jesus quotes Gen.2:24 about a husband and wife becoming one flesh; the word used is hen. Jesus prays that as believers we will be one even as ( Gr. kathos; according as, just as, even as) he and the Father are one. He did not mean our persons would be fused together, it means spiritually united. The God of the Old Testament is a united one. The God of the New Testament is a united one. This is what is meant by God being one.
Jn.10:30 "I and my Father are one," this is not numerical; Jesus is not saying he is the Father. They are not one person, but in nature they are unified. It actually reads "we are one" in Greek the first person plural esmen means we are. Again this is a unity in nature not a numerical statement
The word one in Greek is Hen and it is a neuter nominative so it refers to one in essence and nature, and kind. That he is deity just as the Father is. He went on to explain he is the Son of God and the Pharisees understood his claim of making himself out to be equal with the Father.
Eph. 2:18 " Through Christ we both have access to the Father, by one Spirit."
Here we have three distinct persons involved in our relationship to God. Christ the person (God/man) has made it possible, the Spirit who is the means of access and the Father who is the object. The scripture uses "we" both in relation to the Jews and gentiles, two people groups. These same words are used also of the Son and the Father showing they are two different persons.
2 Cor. 13:14 "May the grace of the Lord Jesus Christ, and the love of God, and the fellowship of the Holy Spirit be with you all". Here again are all three mentioned why make distinctions if they are all the same person.
There are numerous illustrations in the O.T. of the three being one. For example there is a tradition even today as the Jews say the Shma Deut.6:4 it is broken up into three segments each repeated three times while the last word one is said only once, indicating unity. Isaiah when he saw the Lord in his temple Isa.6 said, "holy, holy, holy,". Also in Rev. 4:8 it describes the angels saying it thrice day and night, showing it is a set pattern in heaven where God dwells. Yet more times than not God is represented as the Holy one of Israel. In Numbers 6:24 we find a thrice fold blessing with the title Lord. Num 6:24-27 "The LORD bless you and keep you; the LORD make His face shine upon you, and be gracious to you; the LORD lift up His countenance upon you, and give you peace." "So they shall put My name on the children of Israel, and I will bless them." In like manner 2 Cor.13:14 states" The grace of the Lord Jesus, and the love of God and the communion of the Holy Spirit be with you all." Here we see all three mentioned as a unit. While this cannot emphatically prove tri-unity it does show there was this concept that is not foreign to the scriptures.
The word one clearly is used a united plural and that is the teaching revealed from Genesis to Rvevaltion.
These are excerpts from the book Who is Jesus ? Answering Oneness Pentecostals attacks on the Trinity. spiral book by Mike Oppenheimer of Let Us Reason ministries Wahiawa HI 96786