Jews were also forbidden from giving testimony concerning Christians in a court of law—a restriction already present in the Theodosian code—although Justinian eased this restriction in 537 to allow them to testify in cases between Christian individuals and the state. This privilege was not enjoyed by any other non-Christian group. Once more, the state sacrificed the doctrinal subordination of the Jews in order to gain practical benefits, in this case testimony against those who faced it in court. Questions of internal Jewish discourse—which could, under the Theodosian Code, be arbitrated only by Jewish courts—could, under the Justinian Code, be officiated by the state, a power which Justinian did not shy away from utilizing. In 553 for instance, Justinian required that the public reading of the Pentateuch proceed in vernacular, rather than Hebrew, and forbade altogether the reading of the "Mishna". In this way, Justinian not only restricted the religious freedom of the Jews, but also expanded his own power in order to reinforce the principle that, "in theory, there is no area that falls outside of the Empire's legislative power". Justinian's restrictions were, however, poorly enforced. Ironically, what little enforcement they did enjoy contributed to a notable growth in Jewish culture and liturgy. For instance, the banning of the reading of "Mishna" prompted Jewish scholars to write the "piyutim", important works of poetry which refer strongly to the "Mishna". Because these were not banned by the Civil Code, they afforded Jews the ability to circumvent it. Accordingly, this form of religious expression flourished under Justinian. Although the Justinian Code remained in force in the Eastern Empire until the ninth century, the period following Justinian's reign was generally characterized by toleration of non-Christians, particularly the Jews. However, during the Byzantine–Sasanian War of 602–628 many Jews sided against the Byzantine Empire in the Jewish revolt against Heraclius, which successfully assisted the invading Persian Sassanids in conquering all of Roman Egypt and Syria. In reaction to this, anti-Jewish measures were enacted throughout the Byzantine realm and as far away as Merovingian France. Soon thereafter, in 634 the Muslim conquests began, during which many Jews initially rose up again against their Byzantine rulers. During this time Heraclius became the first emperor to force the conversion of Jews to Christianity. Following his death, and until 1204, the Jews suffered only three notable legal persecutions, the sum of whose span was roughly fifty years.