Emperors Theodosius II and Honorius issue an order for all Caelicolists (apparently a combination of Christianity and Judaism) to return to orthodox Christian beliefs within one year. It is also ordered that if anyone induces Christians to convert to the Caelicolist heresy, they will be punished as traitors.
This new decree states:
"A new crime of superstition has appeared under the name of Caelicolists, until now unheard of. Unless the Caelicolists return to the worship of God and the veneration of Christianity within one year, they will be subject to the laws by which heretics are restrained. For it is certain that if any doctrine differs from the faith of the Christians, it is contrary to Christian law.
Some people, unmindful of their lives and of the law, dare misuse this faith by compelling some Christians to assume the detestable and offensive name of Jews. Those who have committed this crime have rightly been condemned by the laws of previous Emperors, but We will admonish them frequently to ensure that those who have been instructed in the Christian mysteries will not be forced to adopt a perversity that is Jewish and alien to the Roman Empire, after having already adopted Christianity.
If anyone attempts such a course, We direct that the authors of the deed, with their accomplices, to be subject to the penalty provided by previous laws for heretics. It is more grievous than death and more cruel than murder if any Christian is polluted by Jewish disbelief.
We thus command that no one who harms the churches or leads away anyone who, under our regulation, is faithful and devoted to God. If anyone should attempt to contravene this law, they will be found guilty of the crime of high treason." [CTh 16.8.19]
Converting adherents of other religions to Christianity is, of course, completely legal and probably even encouraged. Converting any Christian to another religion, and especially to Judaism, is considered horrible. This is just one of many ways in which the power of the Roman state is used to protect Christianity and Christians. The existence of Jews is tolerated and Jews even receive some protections, but the purity of Christianity is an even higher priority.