During the reign of Constantine many of the pagan holidays were converted to Christian holidays. It was to unite the empire and expand his power. This has the advantage of keeping the holiday which the pagan believer’s liked but allowed the Christians to celebrate also. Everyone was happy in with this tradition.
Many early Christians would not have gone along with Constantine’s conversion if he had not used the names of Christian saints or martyrs for these holidays. After all, Constantine had been a sun worshiper and only after his conversion were temples converted to churches.
By converting these holidays to Christian use, he could convert more of the pagans to the Christian religion. By uniting these factions, his kingdom became stronger. There would have been resistance initially by both parties, but with time conversion became easier.
He built great churches so he would have had an outlay of money that would have been a negative. However, by building these churches he brought the people closer together and would have gained a reputation as a ruler.
There would also have been religious disputes that would have needed the attention of the government. This would have taken time away from conquering lands and could have caused the government’s downfall. He also would have been the head of the church at the time and as such would have been responsible for it. This would have also caused much time to be devoted to the church rather than governing of the kingdom.