The notion which the phrase ‘penal substitution’ expresses is that Jesus Christ our Lord, moved by a love that was determined to do everything necessary to save us, endured and exhausted the destructive divine judgment for which we were otherwise inescapably destined, and so won us forgiveness, adoption and glory. To affirm penal substitution is to say that believers are in debt to Christ specifically for this, and that this is the mainspring of all their joy, peace and praise both now and for eternity.Packer has this to say about "Substitution and divine love":
The penal substitution model has been criticised for depicting a kind Son placating a fierce Father in order to make him love man, which he did not do before. The criticism is, however, inept, for penal substitution is a Trinitarian model, for which the motivational unity of Father and Son is axiomatic.
The New Testament presents God’s gift of his Son to die as the supreme expression of his love to men. ‘God so loved the world that he gave his only-begotten Son’ (John 3:16). ‘God is love, . . . Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins’ (1 John 4:8-10). ‘God shows his love for us in that while we were yet sinners Christ died for us’ (Rom. 5:8).
Similarly, the New Testament presents the Son’s voluntary acceptance of death as the supreme expression of his love to men. ‘He loved me, and gave himself for me’ (Gal. 2:20). ‘Greater love has no man than this, that a man lay down his life for his friends. You are my friends . . .’ (John 15:13f.) And the two loves, the love of Father and Son, are one: a point which the penal substitution model, as used, firmly grasps.Furthermore, if the true measure of love is how low it stoops to help, and how much in its humility it is ready to do and bear, then it may fairly be claimed that the penal substitutionary model embodies a richer witness to divine love than any other model of atonement, for it sees the Son at his Father’s will going lower than any other view ventures to suggest.
That death on the cross was a criminal’s death, physically as painful as, if not more painful than, any mode of judicial execution that the world has seen; and that Jesus endured it in full consciousness of being innocent before God and man, and yet of being despised and rejected, whether in malicious conceit or in sheer fecklessness, by persons he had loved and tried to save — this is ground common to all views, and tells us already that the love of Jesus, which took him to the cross, brought him appallingly low.
But the penal substitution model adds to all this a further dimension of truly unimaginable distress, compared with which everything mentioned so far pales into insignificance. This is the dimension indicated by Denney — ‘that in that dark hour He had to realise to the full the divine reaction against sin in the race.’
Owen stated this formally, abstractly and non-psychologically. Christ, he said, satisfied God’s justice:
for all the sins of all those for whom he made satisfaction, by undergoing that same punishment which, by reason of the obligation that was upon them, they were bound to undergo. When I say the same I mean essentially the same in weight and pressure, though not in all accidents of duration and the like . . .Jonathan Edwards expressed the thought with tender and noble empathy:
God dealt with him as if he had been exceedingly angry with him, and as though he had been the object of his dreadful wrath. This made all the sufferings of Christ the more terrible to him, because they were from the hand of his Father, whom he infinitely loved, and whose infinite love he had had eternal experience of. Besides, it was an effect of God’s wrath that he forsook Christ.And the legendary ‘Rabbi’ Duncan concentrated it all into a single unforgettable sentence, in a famous outburst to one of his classes: ‘D’ye know what Calvary was? what? what? what?’ Then, with tears on his face — ‘It was damnation; and he took it lovingly.’
This caused Christ to cry out . . . “My God, my God, why hast thou forsaken me?” This was infinitely terrible to Christ. Christ’s knowledge of the glory of the Father, and his love to the Father, and the sense and experience he had had of the worth of his Father’s love to him, made the withholding the pleasant ideas and manifestations of his Father’s love as terrible to him, as the sense and knowledge of his hatred is to the damned, that have no knowledge of God’s excellency, no love to him, nor any experience of the infinite sweetness of his love.
It is precisely this love that, in the last analysis, penal substitution is all about, and that explains its power in the lives of those who acknowledge it.