Sinclair Ferguson on substitution and sanctification

In his work, On the Incarnation, Athanasius wrote:

"It would, of course, have been unthinkable that God should go back upon His word and that man, having transgressed, should not die."

How then could man be saved from death, the penalty justly imposed by God for disobedience, except by the death in his place of the sinless Saviour? Without penal substitution the truthfulness of God, his fidelity to his word, is left in tatters. Was God not bound by his own nature to fulfill his word? It was His life for ours.