The reign of Constantine established a precedent for the position of the Christian emperor in the Church. Emperors considered themselves responsible to God for the spiritual health of their subjects, and thus they had a duty to maintain orthodoxy. The emperor did not decide doctrine – that was the responsibility of the bishops – rather his role was to enforce doctrine, root out heresy, and uphold ecclesiastical unity. The emperor ensured that God was properly worshiped in his empire; what proper worship consisted of was the responsibility of the church.
Wikimedia Foundation (2015). Christianity in the 4th Century. Wikipedia. Available http://en.wikipedia.org/wiki/Christianity_in_the_4th_century. Last accessed 18th Apr 2015.