Answer:
Constantine had been a pagan before he converted to the Christianity. After Constantine died, his successor Constantinus decided that Byzantine needed his own Pantheon.
It was a wooden-roofed basilica, built on the site of a pagan temple. On the contrary to the popular belief, it was donated by Constantinus II not by Constantine the Great.
Old pagan temples were descructed and the remains of them were used in new Christian structures.
The basic idea was Jnion through religion.
Constantine was the emperor to make christianity the state religion. By building it in Constantinople it was a monument that said 'one religion for rome'.
