Truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished
So the question becomes, "Why do we still need the Law, if Jesus has come to fulfill the Law through his life, death and resurrection?" If we are not under the Law but under grace, why will not the smallest letter, or the least stroke of a pen not disappear until everything is accomplished. And what does Jesus mean when he says, "until everything accomplished?"
Clearly this shows that we are in a period of time where we still need the Law. And Jesus further adds this time is until, "heaven and earth disappear". We can assume he means at the end of time when he returns to inaugurate a new heaven and earth. I would suggest that there are two reasons that Jesus says to the disciples that even though he is a fulfillment of the Law, it should not in any be abolished until all is accomplished.
1. The Law leads people to Christ. Until all people come to Christ, the Law is still needed to convict sinners why they need Jesus. We know that there are many who have still not heard of Christ, or have not yet heeded the conviction of the Law. God waits for all to repent and turn to Him for forgiveness, and He will do this until Jesus returns.
2. Christians are still guided by the Law, yet not ruled by it. We are no longer slaves to the Law, because we have been set free from it through Jesus' work on the cross delivering us from sin. Since the moral law of the a Old Testament, as summed by the Ten Commandments, doesn't change it is still a faithful guide to our new life in Christ. For example, adultery and stealing will always be wrong, but being saved by grace doesn't mean these laws don't apply to us. The difference is we don't obey these laws to get God's approval, but because we have it.
And as Paul says, "Although I am not perfect, I press on for the goal for which Christ Jesus called me heavenward!" Meaning Paul is saying I don't perfectly measure up to the Law, but inch by inch, day by day, my life is being further conformed to Jesus' image through the work of the Spirit in me.
So when Jesus returns and consummates God's kingdom, we won't need the Law anymore, because we will be set free from this body of sin and death. But for today, let us remember that until every everything is fulfilled, we are still making progress. We are growing to be more and more the type of people who are transformed by God's grace and do the things he would do if he were us. This is but a foretaste of what is to come!!