John 10:30
"The Father and I are one."
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A Jehovah's Witness told me that Jesus was not God, but the Son of God.
Are you a Jehovah's Witness? Or some other non-trinitarian sect?
No, I am not. We believe in the Trinity and use both the Apostle's Creed and the Nicene Creed regularly, and once a year the Athanasian Creed.
Words have meaning. Sometimes they differ slightly from one language to the next; as you know, the Bible was not written in English.
In English, the capitalized word God most universally refers to the entire Triune entity, which in the Catholic Catechism and many other Christian denominations is knows as God in Three Persons.
As John 14:21 notes, Jesus said that the Holy Spirit would be sent by His Father, indicating that another Person of God was not yet present with the disciples. In that passages and many others, Jesus refers to the Father as a separate aspect of His Being while He was here on earth as the Son.
The Christ was sent as fully God, but also incarnated as fully man. Mary was the mother of that man—not of the Triune God in total.