The term Trinity was coined shortly after the biblical period but not the one God formula.
The one God formula came out of the Ecumenical Councils because they could not come up with anything else to settle the disagreements. So this belief is essentially Catholic doctrine.
The McKenzie Bible Dictionary, explains it this way.... “The Trinity of God is defined by the Church as the belief that in God there are three persons who subsist in one nature. The belief as so defined was reached only in the 4th and 5th centuries AD and hence is not explicitly or formally a biblical belief.”
Christ explains the oneness concept like this….
John 17:22 “And the glory which to them; that they may be one just as We are one.” Because Yeshua says “just as” this is an exactness, a duplication of a condition that we can achieve, and He states that this condition of “oneness” can apply to us, but it has nothing to do with absorption or singularity, but rather a condition of spiritual union and solidarity between God and us.
The next verse further defines this by describing a unity with Christ that would cause the same condition with us as it did with them, a condition of perfection. Again, not talking to Himself, in John 17:23 “I in them, and You in Me, that they may be perfected in unity, that the world may know that You did send Me, and do love them, even as you do love Me.”
In this context millions of people could be made one...one being a abstract concept of one, but a more literal meaning of unity, solidarity, and perfection and even a “body” that is considered one....the body of Christ or the body of the Church.
And then, the next verse is probably one of the best verses to put this oneness concept into perspective. The leading verses are speaking of the works of the Holy Spirit and then ends with this explanation. 1st Corinthians 12:11-13 “But one and the same Spirit works all these things, distributing to each one individually just as He wills. For even as the body is one and yet has many members, and all the member of the body, though they are many, are one body, so also is Christ. For by one Spirit we were all baptized into one body, whether Jews or Greeks, whether slaves or free, and were all made to drink of one Spirit. For the body is not one member, but many.”