So, when commenters suggest that the Bible really doesn't teach the PST of the atonement, their argument is with their fellow evangelicals Christians who doubt that one can even be a "true" Christian while rejecting the PST.
Listen to D. Martyn Lloyd Jones:
[U]nless you believe that man, by nature, is dead in trespasses and sins, and that there is such a thing as the wrath of God upon sin, whatever else may be true of you, you are not in this holy temple in the Lord, in which God dwells . . . But thank God it does not stop at that, it goes on to tell us about the grace of God . . . the Lord Jesus Christ, His Person and His work . . . it is by His death, by His sacrificial death, by His substituting Himself for us to bear the punishment of our sins, that we are saved. It is by the blood of Christ!
. . . [T]here are people who call themselves Christians who scoff at it. There are leaders in the big denominations who say that it is scandalous to talk about a substitutionary atonement. And I am asked to be one in fellowship with them. How can I be? It is impossible. I have no choice; this is fundamental. The blood of Christ! ‘He bore my sins in his own body on the tree.’ It is by that alone that I am delivered, and by the power of God in regeneration, and the gift of the Spirit (God’s Way of Reconciliation, 1972, p. 347).
While somewhat more moderate, listen to Garry Williams:
[T]he critics need to say that they do believe in penal substitution itself and just not in warped forms of it. But if the accusation is indeed an accusation against penal substitution itself, as I suspect it is, then I fear that we cannot carry on as we are. As much as I would like to, and mindful of the injunctions of the Lord Jesus Christ himself to seek peace, I find it impossible to agree that this is just an intra-mural, within-the-family dispute, when it has been acknowledged by all parties that we are arguing about who God is, about the creedal doctrine of the Trinity, about the consequences of sin, about how we are saved, and about views which are held to encourage the abuse of women and children. So long as these issues are the issues, and I believe that they have been rightly identified, then I cannot see how we can remain allied together without placing unity above these truths which are undeniably central to the Christian faith. I say this with a heavy heart ("Justice, Law, and Guilt – Evangelical Alliance Symposium on Penal Substitution", 2005).
Also Simon Gathercole:
In the case of substitution, however, it seems that the combination of the Bible’s clarity on the issue … and the fact that it is an essential requirement for assurance means that it is not a legitimate area of disagreement amongst Christians ("The Cross and Substitutionary Atonement" in Scottish Bulletin of Evangelical Theology 21 : 155).