What was the first of two important changes that marked emperor constantine's rule?

During Emperor Constantine's rule,
1. Persecution of Christians stopped.
2. The worship of the Roman emperor as God stopped. Instead, the ruling church bishop was called the Vicar of Christ (the Pope).

Emperor Constantine converted to Christianity around 300 AD.
Whether the conversion was genuine or not is a matter of controversy for many reasons, including:
(a) the introduction of many pagan roman practices into a state sponsored church,
 (b) The veneration of statues and of Mary (idolatry),
(c) the formation of distinctions between the clergy and laity (Nicolaitanism).

The state sponsored church was the church of Rome, which is known today as the Roman Catholic church, with a counterpart in the Eastern Orthodox church.

Democracy and darwinism would be my answer.