This week I was on a trip to Rome. When I was there I also visited the Vatican City where I bought my first Vatican stamp sets.
One stamp set was this:
1700 years Edict of Milan (issued 12-06-2013)
(It is a joint issue with Italy)
The Edict of Milan was closed in 313 AD in Milan, the place were the daughter of Constantine (emperor of the west) married Licinius (emperor of the east). The Edict of Milan assured the freedom of religion.