Mungo said:
The problem with Penal Substitution is four fold.
1. The punishment for sin is eternal separation from God – but Jesus is not eternally separated from God.
If the Lord Jesus were an ordinary man, you would have a point but He is the One in whom,
'All the fullness of the Godhead lives bodily' (Colossians 2:9). Turretine wrote,
'Christ alone ought to be estimated at a higher value than all men together. The dignity of an infinite person swallows up....all the infinities of punishment due to us.' Our Lord's sufferings, though they lasted only a finite time, were infinite in value because He is infinitely worthy.
2. God would be legally punishing an innocent person for the sins of another.
All the sins of His people were laid upon the Lord Jesus and He bore them willingly for us (Isaiah 53:6; 1 Peter 2:24; John 10:17-18). On the cross, He was personally innocent, but judicially guilty of every sin committed by His people down the ages.
3. If the legal debt has been paid then no-one can be condemned for sin since then God would be taking double payment for the same debt.
Here you have the problem with a general atonement. Christ laid down His life for the sheep, not for the goats (John 10:11).
4. There is no need for God’s mercy since the debt has been paid. Mercy implies reduction or “letting off” of some or all the debt of punishment.
God's mercy is revealed in His giving of Himself in the Person of His Son to suffer the death, punishment and curse due to fallen humanity as the penalty for sin. If that is not mercy, I don't know what is!
Moreover the whole theory introduced conflict into the Godhead
No it doesn't. The Father sends the Son; the Son does not send the Father. Does that introduce conflict into the Godhead? The Son obeys the Father, prays to the Father; the Father does not obey or pray to the Son. Does that introduce conflict? The Son gets tired and needs to sleep (Mark 4:38); God never grows weary (Isaiah 40:28). Does that introduce conflict? Read John 10:15-18. Does that introduce conflict? For a more detailed response, here's something I wrote a while back in response to just this sort of argument:
Penal Substitution and the Trinity
The whole idea is grotesque.
On the contrary, it's the most wonderful thing I ever heard of! I praise God for it every day.