On the basis of faith (the New Covenant....i.e., God's righteousness manifested apart from the law).On what basis did God forgive guilty sinners?
You keep looking at it like the Father is judging the World via the law BUT Scripture tells us that all Judgment has been given to the Son.
If you are open to God then I encourage you to try and set aside theology and just prayerfully read Scripture. God does not need man's additions and theories to make His Word make sense. He will guide you to truth and maturity.
If you do this I think you will grow to realize just how much the Penal Substitution Theory twists Scripture.