Tertullian, a Latin theologian who wrote in the early 3rd century, is credited with using the words "Trinity", "person" and "substance" to explain that the Father, Son, and Holy Spirit are "one in essence—not one in Person"
About a century later, in 325, the First Council of Nicaea established the doctrine of the Trinity as orthodoxy and adopted the Nicene Creed, which described Christ as "God of God, Light of Light, very God of very God, begotten, not made, being of one substance (homoousios) with the Father".
Since the beginning of the 3rd century the doctrine of the Trinity has been stated as "the one God exists in three Persons and one substance, Father, Son, and Holy Spirit.
Some scholars dispute the authenticity of the Trinity and argue that the doctrine is the result of "later theological interpretations of Christ's nature and function."[26][27] The concept was expressed in early writings from the beginning of the 2nd century forward, and other scholars hold that the way the New Testament repeatedly speaks of the Father, the Son, and the Holy Spirit is such as to "compel a trinitarian understanding of God".
I refer you to my previous answer. It is merely an archaic grammatical construct.
Your most recent reply is self-defeating, of course. Either the Bible is a truthful record or it is not.
By finding errors in one part of it you demonstrate that the rest of it cannot be considered reliable. So using the Old Testament to disprove the New simply discredits the entire Bible.