"Hence it is no longer for Christians to follow the law "
sure, you or anyone else, perhaps with christian or non christian label, don't have to follow any cosmic law, but that doesn't mean that if you don't then you will be exempt from the consequences of not working with it.
what some believe, and what actually happens, are often two quite different things.
"In the same way a Christian can find it hard to believe they have a new nature that is good."
human nature is something that we create ourselves if you examine yourself, you will find that there are factors that are of a good nature and other factors that are of a bad nature. it is entirely your responsibility. you are accountable for the consequences when you shut the car door on your fingers. if there was such a person, reading between the lines, jesus seemed to give a fair bit of instruction on how to clean up your oun mess.
"If any man will come after me, let him deny himself, take up his cross, and follow me." Matthew 16:24
