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In this case we refer to humanity and Divinty.
"For God so Love the world that he sent his own begotten son, that whosoever believe that in him should not perish, but have everlasting life." - John 3:16
Humanity- this is where God came down to earth in the flesh just as man is made of flesh. He went through the same procedure where we came into the world being in our mothers womb for 9 months then being born. He appeared in flesh to redeem us from our sins; he took on our sin. Why? Because he couldn't depend on anyone to do the work so he came down and did it for himself.
Divinity - God is a spirit being he has no hand but our hand and he has no mouth but our mouth. He didn't want to come down as he is cuz as his word said no man shall see the face of God and live. We cannot behold his Glory.
To sum everything up Jesus and God are ONE because he Jesus is God in the Flesh. He is only called son because just as we are sons and daughters to our earthly father. Being that he wanted to show us , for us to follow in his foot steps and show us the way we should live he took on sonship.