How do we know that the atonement is finished?

In John 19:30, Jesus proclaimed 'It is finished' just before His death, emphasizing that the penalty for sin had been fully paid. This declaration signifies the completion of the sacrificial system, where no further offerings are needed for sin. As preached, this work is foundational: 'He hath made reconciliation for the sins of the people,' which reinforces that Christ's sacrifice was sufficient and complete. The implications of this finished work are profound, namely, that believers are fully accepted by God through Christ’s atoning sacrifice.