Does salt lose its saltiness?

     Our series on the Sermon on the Mount continues this week with three separate 'lessons' from Jesus.  The first is from Matthew 5.13, "You are the salt of the earth," the second is Matthew 5.14-16, "You are the light of the world," and the third is Matthew 5.17-20, "[Jesus] has come not to abolish the law, but to fulfill."  I think that we are fairly familiar with the first two sayings.  Salt of the earth and light of the world are often used to describe Christians and the Church, in fact, we often refer to individuals as 'salt of the earth people.' 
     I am intrigued by Jesus' third saying.  He says that he has not come to abolish the law, but to fulfill it.  He then goes on to say, "therefore, whoever breaks^* one of the least of these commandments, and teaches others to do the same, will be called least in the kingdom of heaven; but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven."  I'm not sure about you, but in reading the law (often called the Torah or Pentateuch, they are the first five books of our Bible) we know that it is impossible to fulfill every letter of the law.  The law was given as a gift to God's chosen people after God had already made the covenant with them.  Staying in right relationship with God was dependent God's people being righteous - being who God created them to be.  We know from reading time and time again in our Hebrew Bible that God's people could not be righteous and kept turning from God.  So God in Jesus has fulfilled what we cannot: right and perfect relationship with God.
     On the cross, Christ's righteousness becomes our own righteousness.  It is, as Luther says, an alien righteousness, something given to us totally outside of ourselves.  It is not anything we can earn or accept, but we can be only passive recipients of Christ's righteousness.  With our righteousness being that of Christ’s, surely our righteousness will exceed that of the Pharisees and Scribes, as Jesus says in verse 20.  What a gift!  Because of Christ’s righteousness we are able to enter the kingdom of heaven – not only after death, but right now, in the present!  God’s kingdom is here on earth in bits and pieces, and we are blessed to experience it every now and then.  Right relationship with God, reconciliation with neighbor, living as salt and light, these are all signs of the kingdom come. 
     We give thanks for this alien righteousness, but it does leave me with the mystery: if Christ’s righteousness is our own righteousness, then how does salt lose its saltiness?