Salt only works when applied. Christians must not retreat from culture--trying to preserve our own purity. The Holy Spirit preserves us by the Grace of God. Retreating from our culture, we lose our effectiveness. What use is salt if it loses it's flavor? Jesus tells us we must salt the world, bringing flavor to the lost around us; giving them a taste of the His righteousness and a glimpse of His glory accomplished in part by our obedience to the great command: "Go into all the world..." And, leaving them with a hungering for more of Him and His righteousness.
Notes from Apologetics and Outreach, Lecture 10.