There is only one God the Father and only one Son of God, Jesus, and one Holy Spirit.  These add up to three but there is only one God, who is the Father of Jesus.  When the bible says these three are one, it means they are in agreement. Jesus did not some how swallow up God the Father and cause him to cease to exist.

In the Old Testament in different places, it says the people saw God.  But in the New Testament Jesus says, "No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him. (ST John 1:18)  If Jesus was God, why did he say, "No man hath seen God at any time"  because the bible says different people had plainly seen God at times in the past and he was standing in plain sight right in front of them when he said, "No man hath seen God at any time." It is obvious that Jesus was saying, "The one you saw, who you thought was God, was really me."

God of the Old Testament

Jesus is called God a few times in the Bible and was received as God in Old Testament times. The apostle Paul said of Jesus:

Philippians 2:6

"Who, being in the form of God, thought it not robbery to be equal with God:"

Hebrews 1:1-3

1 God, who at sundry times and in divers manners spake in time past unto the fathers by the prophets,

2 Hath in these last days spoken unto us by his Son, whom he hath appointed heir of all things, by whom also he made the worlds;

3 Who being the brightness of his glory, and the express image of his person, and upholding all things by the word of his power, when he had by himself purged our sins, sat down on the right hand of the Majesty on high;
 

It was understandable how Jesus could be received as God in that the Father had given Jesus great power, authority, and glory.  But the full revelation of truth came when Jesus explained how his Father had sent him and appointed him as savior, judge and ruler of all mankind and angels. The idea that God had a Son is basically a New Testament revelation. The concept of the "Son of God" is hardly mentioned in the Old Testament.

Jesus Will Always Be Subject to the Father

The Apostle Paul speaking of the resurrection and future events says Jesus will always be subject to God the Father.

1Corinthians 15:27-28

27 For he (God the Father) hath put all things under his (Jesus) feet. But when he saith all things are put under him, it is manifest that he (God the Father) is excepted, which did put all things under him (Jesus).

28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him (God the Father) that put all things under him, that God may be all in all.

Paul says it is manifest or plain and easy to understand (by anyone with common sense) that the Father who exalted and blessed His Son is not under the feet of His Son, but still has the final authority.

Names of God

The bible shows that God the Father and Jesus have shared the same name since the beginning of time. Their Old Testament name was Jehovah. Their New Testament name is Jesus. This is why Jesus said to baptize people in the name of the Father and of the Son and of the Holy Spirit. The Holy Spirit is also called the Spirit of Christ or Spirit of Jesus. There is yet another name change planned, found in Revelation 3:12 "I will write upon him my new name."

One With God

Notice in the verse following how Jesus uses the word one to describe his relationship to his Father.

ST John 10:30

Jesus said, "I and my Father are one."

Then Jesus uses the same word, one ,to describe the relationship of people to one another and the relationship of God to the people.;

ST John 17:11

And now I am no more in the world, but these are in the world, and I come to thee. Holy Father, keep through thine own name those whom thou hast given me, that they may be one, as we are.

ST John 17:21-23

21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.

22 And the glory which thou gavest me I have given them; that they may be one, even as we are one:

23 I in them, and thou in me, that they may be made perfect in one; and that the world may know that thou hast sent me, and hast loved them, as thou hast loved me.

Jesus said we the people should be one with each other as Jesus is one with the Father. Because we the people are one does not mean we cause one another to cease to exist.  In the same way Jesus does not cause God to cease, as Oneness doctrine teaches, but rather is obedient to the Father. The word translated one in these verses is the same Greek word each time.

John taught doctrine of both Father and Son

These verses following show John's doctrine of both the Father and Son:

1John 2:22-24

22 Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son.

23 Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also.

24 Let that therefore abide in you, which ye have heard from the beginning. If that which ye have heard from the beginning shall remain in you, ye also shall continue in the Son, and in the Father.

2 John 8-9

8 Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward.

9 Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son.
 

Oneness doctrine denies the concept of both Father and Son therefore the bible declares oneness doctrine to be heresy.

The next verse shows how Jesus stands between God and men.

1Timothy 2:5

For there is one God, and one mediator between God and men, the man Christ Jesus;

There has to be someone on each side for you to stand between.  You can't stand between yourself and another person.

Heresy

The word “heresy” means: Any belief different from or contrary to what is taught in the Holy Bible.
 

The bible says you should reprove heresies and if the people do not repent, then reject them. (Titus 3:10) The Apostle Paul did not preach oneness doctrine and said, “If anyone preaches any other gospel unto you than that which we have preached unto you, let him be accursed.” (Galatians 1:8)   And in Galatians 5:19-21 Paul lists heresies among other sins such as witchcraft and murder and says, "they which do such things shall not inherit the kingdom of God."

Galatians 5:19-21

19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, 20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, 21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God