Show them the priest's confession from the Coptic liturgy of St. Basil:
Amen. Amen. Amen. I believe, I believe,
I believe and confess to the last breath;
that this is the Life-giving Body that
Your Only-Begotten Son, our Lord, God
and Savior Jesus Christ took from our
lady, the lady of us all, the holy Mother
of God, Saint Mary.
He made It One with His divinity without
mingling, without confusion, and
He witnessed the good confession
before Pontius Pilate. He gave It up for
us upon the holy wood of the cross, of
His own will, for us all.
Truly, I believe that His divinity parted
not from His humanity for a single
moment nor a twinkling of an eye; given
for us for salvation, remission of sins
and eternal life to those who partake of
I believe, I believe, I believe that this is
so in truth. Amen.
If they keep insisting on their blasphemous and wrong understanding, either walk away or punch them in the face, depending.*
* Only punch them in the face if they are Arius.