Who died on the cross? Spirit or flesh?

 

"And as Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, 15 that whoever believes in Him should not perish but have eternal life."

Not sure if you know the story with the serpent but when Israel was taken out of bondage, they complained while in the dessert and sinned against God. So God sent serpents into their midst. People got bitten and died. 

So people repented of their sins against God and ask Moses to speak to God. 

So God told Moses to make a serpent and stick it on top of a pole. God said that whoever gets bitten should look up to the serpent to be healed.

And the same way the son of man Jesus had to be lifted up.

 And the people spoke against God and against Moses: “Why have you brought us up out of Egypt to die in the wilderness? For there is no food and no water, and our soul [c]loathes this worthless bread.” 6 So the Lord sent fiery serpents among the people, and they bit the people; and many of the people of Israel died.
7 Therefore the people came to Moses, and said, “We have sinned, for we have spoken against the Lord and against you; pray to the Lord that He take away the serpents from us.” So Moses prayed for the people.
8 Then the Lord said to Moses, “Make a fiery serpent, and set it on a pole; and it shall be that everyone who is bitten, when he looks at it, shall live.” 9 So Moses made a bronze serpent, and put it on a pole; and so it was, if a serpent had bitten anyone, when he looked at the bronze serpent, he lived. Numbers 21

This means:

We have been "bitten" by such a serpent, or rather, we are dying because we have sinned against God. We look to the cross (pole) and see our sinful flesh as the cause of our imminent death (Just as the Israelites looked upon the serpent and remembered why they were dying) 

But who did they have to remember/believe on to be healed/forgiven? God.

So they looked upon the serpent that brought death but believed in God to be healed/forgiven. 

It is the same with Jesus, who came in the likeness of sinful flesh Romans 8.3.

For what the law was powerless to do in that it was weakened by the flesh, God did by sending His own Son in the likeness of sinful flesh/man, as an offering for sin. He thus condemned sin in the flesh, 

We look upon the "son of man" (because we are a son of man) and remember that we are dying in this flesh, but we believe upon Jesus the Christ (who is Spirit) to heal us. 

Now the Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. 18And we, who with unveiled faces all reflect the glory of the Lord, are being transformed into His image with intensifying glory, which comes from the Lord, who is the Spirit  2 Corinthians 3

 Jesus is not two natures but ONE SPIRIT. He came in a flesh like ours so when people look upon Him on the cross,  they are reminded what brings them death (the sinful flesh) but they believe in Jesus Christ to be healed/salvation.

If Jesus Christ was just a mortal created human being He would just stand for the cause of our death (the sinful flesh) but He would not be the solution/salvation.

Jesus Christ is God Almighty in the flesh body.