Why is the concept of once important for Christians?

The phrase 'once' signifies the completeness of Christ’s work on the cross, as seen in Hebrews 9:26. This doctrine dispels the need for continual offerings or sacrifices, which is a key distinction between biblical Christianity and the erroneous views found in some traditions, such as the Catholic doctrine of the mass. In essence, understanding that Jesus' sacrifice is finished assures believers that their sins are fully dealt with, providing peace and a foundation for faith in their salvation.