As the disciples listened to Jesus, they were possibly saying within themselves, How hard it is to enter into the kingdom!
Jesus consented to their thoughts as he further said that to be a faithful follower of his meant to exercise rigorous self-discipline. He had just mentioned the judgment of sin in no uncertain terms. He spoke next of purification from sins as he said, Everyone will be salted with fire (Mark 9:49). Purification of thought and purpose and motive is essential if the servants of Christ would, in turn, have a wholesome and redemptive effect on the world around them. Unless the Christian is purged from sin, he is like salt without savor; that is, his life is ineffective.
This passage in Mark's Gospel recalls the words of Jesus in the Sermon on the Mount: You are the salt of the earth (Matthew 5:13). To fulfull our ministry as Christ's agents of redemption in the world, we must be completely devoted to the Lord. Jesus concluded his discourse concerning true greatness by saying that the disciples could not be the salt of the earth unless harmony prevailed in the fellowship - Have salt in yourselves, and be at peace with each other (Mark 9:50)