396. There are other conditions necessary that a custom may acquire the force of law. (a) Since a law is an ordinance knowingly imposed by the will of the legislator, a custom does not constitute a law if it is followed through the erroneous conviction that it is already a law, or if there is nothing to indicate a will to make it obligatory. Signs of the intention to raise a custom to the dignity of a law are the punishment of transgressors of the custom, the observance of the custom even at the cost of great inconvenience, the opinion of the good that it should be followed, etc. (b) Since a law cannot prescribe except what is reasonable and for the common good, a practice opposed to the Natural or Divine Law, or expressly reprobated by written law as an abuse, or one that is injurious to the welfare of the community, cannot become unwritten law through custom. 397. There are special conditions in order that a custom may do away with an existing law. (a) A written law is not repealed unless the legislator wills to take away its obligation, and hence desuetude or a custom contrary to law does not abrogate a law unless it manifests a purpose not to be obligated by what the law prescribes. This it does if the whole people regard a certain law as a dead letter, or feel that circumstances or the common welfare require the opposite of what the law requires, and have no scruple in acting uniformly according to this conviction. (b) A written law is not repealed, if it is immutable, or if a change would be prejudicial to the common interest; similarly, therefore, a custom cannot abolish a law, unless this law is one that can be abrogated by human acts, and that is not essential to the public good. Hence, customs contrary to the Commandments or to the Law of Christ, customs that are expressly condemned in Canon Law as corruptions, customs that encourage lawlessness or afford occasions of sin, can never do away with a law, no matter how long or by how many they are practised. 398. Those who start a custom contrary to law are sometimes in good faith, and hence are not guilty of disobedience. (a) It may be that they are in ignorance of the law, but have the interpretative will not to be bound by it; (b) it may be that they know the law, but sincerely think that, on account of conditions, it has ceased of itself. 399. Even when a custom has been started in bad faith, it may continue through good faith, and so become not a violation, but an abrogation of the law. Changed conditions may make the law useless or harmful; or the very fact that it is no longer observed may make it too difficult to enforce. 400. Today customs do not so often attain the force of law. Moreover, so difficult is it to know whether any custom has all the qualities necessary for establishing, modifying, or abrogating a law that only an expert is competent to judge in this matter.