What is substitutionary atonement?A Scripture-grounded answer about substitutionary atonement

Substitutionary atonement is the doctrine that Jesus died in the place of sinners, bearing the penalty they deserved. The 'penal' aspect means he bore the legal penalty for sin; the 'substitutionary' aspect means he did so as a substitute — in the sinner's place. Isaiah 53 is the key Old Testament prophecy: 'He was pierced for our transgressions; he was crushed for our iniquities.' This is the dominant atonement theory in Protestant theology, though other models (Christus Victor, moral influence, ransom) also capture biblical themes.