1. It is not an “atonement” but a “propitiation” it is the difference between putting a blanket over vomit on the carpet or having the carpet made clean as if it was never messed up. Propitiation is the idea of a ransom given to pay for our sins so that we do not have to collect the “wages” of sin being death.
2. The gospel of Mark is about Christus Victory. To state that that the only view is substitutionary atonement means we need throw out that Gospel as it does not teach that idea, but it teaches the idea of the “suffering King”.
3. Atonement is an Old Testament and old covenant view. “Taking away the sins of the world” is the NT teaching and view. Jesus not only covers us, but also cleanses us… or as Hebrews states:
“Hebrews 9:13-15 “The blood of goats and bulls and the ashes of a heifer sprinkled on those who are ceremonially unclean sanctify them so that they are outwardly clean. How much more, then, will the blood of Christ, who through the eternal Spirit offered himself unblemished to God, cleanse our consciences from acts that lead to death, so that we may serve the living God! For this reason Christ is the mediator of a new covenant, that those who are called may receive the promised eternal inheritance—now that he has died as a ransom to set them free from the sins committed under the first covenant.”
It seems funny that to teach just “penal atonement only" is to them more “biblical” as you need deny major passages of scripture to make that a “truth”.