Before [the Twelve Tables of the Decemvirs], Roman law had been a mixture of tribal customs, royal edicts, and priestly commands. “Mos maiorum“–the way of the ancients–remained the exemplar of morals and a source of law … early Roman law was a priestly rule, a branch of religion, surrounded with sacred sanctions and solemn rites. The priests declared what was right and wrong (“fas et nefas“), on what days the courts might open and assemblies meet. All questions regarding marriage or divorce, celibacy or incest, wills or transfers, or the rights of children, required the priest as now so many of them require the lawyer. Only the priests knew the formulas without which hardly anything could be legally done. The laws were recorded in their books, and these volumes were so securely guarded from the plebs that suspicion charged the priests with altering the texts, on occasion, to suit ecclesiastical or aristocratic ends.

The Twelve Tables effected a double juristic revolution: the publication and secularization of Roman law.

–Caesar and Christ, by Will Durant, p. 31