Without substitution the death of Jesus is unintelligible. Unless what we have here is what is being described in 2 Corinthians 5:21, that he was made sin for us–not that he was made a sinner for us–but made sin for us, then how else do you explain it? What possible justification could God have for crucifying the innocent unless in substitution he became all that we are in our sin and rebellion in order that, in the mastery and mystery of his grace, in him we might become the very righteousness of God? … There is no story in all of human history like this. There is no notion in all religions of the world that comes close to touching this. This is imponderable, mysterious, majestic, glorious. This is all about God and the wonder of his grace.Alistair Begg
The Oxford English Dictionary defines substitution as ‘the putting of one person or thing in the place of another’. One oddity of contemporary Christian talk is that many who affirm that Jesus’ death was vicarious and representative deny that it was substitutionary; for the Dictionary defines both words in substitutionary terms! Representation is said to mean ‘the fact of standing for, or in place of, some other thing or person, esp. with a right or authority to act on their account; substitution of one thing or person for another.’ And vicarious is defined as ‘that takes or supplies the place of another thing or person; substituted instead of the proper thing or person.’ So here, it seems, is a distinction without a difference. Substitution is, in fact, a broad idea that applies whenever one person acts to supply another’s need, or to discharge his obligation, so that the other no longer has to carry the load himself. … In this broad sense, nobody who wishes to say with Paul that there is a true sense in which ‘Christ died for us’ (huper, on our behalf, for our benefit), and ‘Christ redeemed us from the curse of the law, having become a curse for us’ (huper again) (Rom. 5:8; Gal. 3:13), and who accepts Christ’s assurance that he came ‘to give his life a ransom for many’ (anti, which means precisely ‘in place of’, ‘in exchange for’), should hesitate to say that Christ’s death was substitutionary. Indeed, if he describes Christ’s death as vicarious he is actually saying it.J. I. Packer, What Did the Cross Achieve?