Mark 9:49

For every one shall be salted with fire, and every sacrifice shall be salted with salt.

In the worship of that time every sacrifice presented was treated with salt before it was offered. That salt meant that no human service is acceptable to God on its own, but only as it is made righteous through Christ. Those who give themselves as living sacrifices must therefore receive this preserving salt of Christ's righteousness, and by His Spirit and grace they themselves become the salt of the earth that holds back evil in the world. A mere profession of godliness, without the love of Christ in the heart, has no power to do good and exerts no saving influence upon others.