Constantine (272-337), who reigned from 306-337, was the first Roman Emperor to publicly profess to hold to Christianity.
During his reign, when his co-emperor Licinius persecuted the church, Constantine waged war with him to liberate the Christians. The following is written in "A Defense of Liberty Against Tyrants" on this matter:
Constantine and Licinius governed the empire together, the one in the Orient, the other in the Occident. They were associates of equal power and authority. And amongst equals, as the proverb is, "There is no command."
Notwithstanding, because Licinius does everywhere banish, torment, and put to death the Christians, and amongst them divers of the nobility, and that for and under presence of religion, Constantine makes war against him, and by force compels him to give free liberty of religion to the Christians; and because he broke his faith, and relapsed into his former cruelties, he caused him to be apprehended and put to death in the city of Thessalonica. This emperor's piety was with so great an applause celebrated by the divines of those times, that they suppose that saying in the prophet Isaiah to be meant by him: "That kings shall be pastors and nursing fathers of the church."
 Junius Brutus, Vindiciae Contra Tyrannos: A Defense of Liberty Against Tyrants: Question Four: Whether neighbor princes may, or are bound by law to aid the subjects of other princes. (Amsterdam: Valckenier, 1660). Attributed to Philippe Duplessis-Mornay (1549-1623) and Hubert Languet (1518-1581). Thanks to Kevin Gowen for work on revisions, mainly to modernize some of the language. Accessed from the Constitution Society.
Note about the Theonomy Applied Series: In quoting any particular law, we do not necessarily endorse every aspect of that law as biblical, whether it be the prohibition, sanction, court procedure, etc. Rather, we are merely showing the more or less attempt to apply biblical law in history, whether or not that application was fully biblical. Moreover, in quoting any particular law, we do not necessarily consider those who passed and/or enforced such a law as being fully orthodox in their Christian theology. Professing Christian rulers in history have ranged in their theology from being orthodox (that is, Reformed Protestants) to heretical (for example, Roman Catholics).