This statement does not preclude justice. It does not mean that Christians have no rights. Those rights given to us by the Lord and guaranteed by the Constitution define a framework in which society should operate. Government is designed to secure those rights and enforce penalty upon those who violate those rights.
So, though Christians should at the individual level operate in love and mercy, it is not practical for government to operate in such manner. If it did, there is no government. There is only lawlessness and anarchy.