This means that while under the law faith was NOT in operation. Jesus Christ is the faith that was to come. The law was our tutor, our guardian responsible for the care and discipline of the children. After faith, Jesus Christ, came we are no longer under the law. Therefore we need to ask this question concerning the law.
Lets seek the answer in the next point.
We teach it because we are still being deceived by sin. Before God added the law, man was already a sinner. The law was not added to cause man to become a sinner but rather to allow man to know that he was a sinner. Sin and law are two separate issues. Before the law came man was sinning.