Then why the Law?. ( Romans 7 )

If no one can keep the law, why was it given by God? to make what sin is,as Paul the apostle writes ,'I would not have known what coveting really was if the law had said,'you shall not covet'. ( Romans 7 v 7) .This is a very important reason why ministers must not avoid spelling out clearly,what is a sin,the morality of the world can be a very flawed thing, and often goes contrary to what Gods word declares as sin.I remember saying that a certain thing was a sin ,and was told that's your opinion.No it was not just my opinion ,it was based on what God's word said, the person who said this was a minister.When God's word declares that something is a sin,it should not be a matter of opinion, it is a sin.As Christians we must not lean on our own understanding,or the worlds understanding ,as regards morality.