What did Jesus mean when he said ‘Go and sin no more’ if sinlessness is impossible? I came across these two instances in the New Testament when Jesus told someone to "sin no more."


  John 13:15 Now the man who had been healed did not know who it was, for Jesus had withdrawn, as there was a crowd in the place. Afterward Jesus found him in the temple and said to him, “See, you are well!  Sin no more,  that nothing worse may happen to you.” The man went away and told the Jews that it was Jesus who had healed him.  


The second instance is in the account of the woman taken in the act of adultery.


  John 10:11 Jesus stood up and said to her, “Woman, where are they? Has no one condemned you?” She said, “No one, Lord.” And Jesus said, “Neither do I condemn you; go, and from now on sin no more.”  


From the standpoint of sinlessness being impossible, what exactly did Jesus mean by saying this?
When you look at Jesus' command in light of the whole law, and other instances where He said similar things (such as "be ye as perfect as your father in heaven"), you realize that while this is a command, it's really just a repetition of all the previous commands given by God over thousands of years.  Jesus' command is just a reaffirmation of what God has been commanding us since the beginning of time - or at least of the law.

In a sense, it appears that God is commanding the impossible.  And one wonders why he would do that.  What does he expect to get from all this commanding.  Well, the truth is that the Law, while good, was never intended to be a guide for how to live one's life.  It's purpose is to convict of sin, "that the whole world might become guilty before him".  Why would he do this?  Because the Law of God is not a guide for living life.  it is not a list of things that you MUST do, it is a list of things that you do NOT do, and in not doing them you have become guilty before God.  The purpose of the Law, again, is not lifestyle guide, but schoolmaster - leading man to the cross of Christ.

Because the plan of salvation depends on the works of Jesus Christ, not man.  Man cannot even contribute to this work, for, "if you will build me an alter, you shall do it with uncarved stone (untouched by man), for in the moment that you lift up your tool upon my altar, you have polluted it."

This plan of salvation makes it possible for men to come to God through Christ, but the condition is that you must first be found guilty, in order to take advantage of the court's mercy (Christ).

Jesus' command to "go and sin no more" is simply a reaffirmation that "God is Holy, and Man is Sinful,... flee the wrath to come".  That once you trust Him for salvation, you will always, and must always continue to trust Him and His work done on your behalf, not in your own works.