Per the Nicene Creed:
Of one being with the Father
Per the Holy Bible:
Don’t you believe that I am in the Father and the Father is in me? The words I speak are not my own, but my Father who lives in me does his work through me. Just believe that I am in the Father and the Father is in me. Or at least believe because of the work you have seen me do.
I have given them the glory you gave me, so they may be one as we are one. I am in them and you are in me. May they experience such perfect unity that the world will know that you sent me and that you love them as much as you love me.
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Truth is...If it helps to think of deity as being a physical substance, then we can say that God the Father and God the Son (stay tuned for truth about God the Holy Spirit) are both made up of the same "stuff".