I'm not sure what you mean by 'other' Christians but I'll try to answer what I think you mean.
I think it is basically due to the Old v New testaments and the Law v the New Covenant through Jesus Christ. The OT serves to teach us that man cannot follow the rules required by God and we are, as such, all sinners. That is where Jesus comes in: As sinners, the only way we can be reconciled is through accepting Him as our savior so that he can take away our sins. That does not mean that we are now free to break whatever rules we feel like, rather we still need to strive to do as God desires.