This was the new direction of Constantine in taking the Roman society in the direction of Christianity. 

There was a new capital and a new look of art and architecture leaning initially on Roman archetypes and then branching out to hexagonal, round and buildings in a course cross shape with a dome in the center.

It was also intended to include an Eastern influence to unite both the East and West in a common style.   

The Churches were plain on the outside (pious) and luxurious on the inside

They had expanded from a rectangular structure different shapes, including one in the form of a cross.

Entering into one of these structures, the parishioner entered into the symbolic state of the crucifixion, or the universe.

The parishioners a new style of building, to better help them to understand the significance of the crucifixion, the sacrifice, the purpose of celebrating and glorifying this, as most were not educated in this new concept 

The images of icons within the Churches serves the same function, to help a poorly educated populous relate to the human aspects of the religion

The churches were generally small, as the population was generally small, built of simple brick or stone