Emperor Constantius II issues a new law exempting clergy from compulsory public service.
Constantius' new law says:
"All clergy are exempt from compulsory services as decurions and from every annoyance of municipal duties. Their sons must remain with the Church to not become obligated to the municipal councils." [CTh 16.2.9]
A "decurion" is a local administrative official. In addition to monetary taxes, the Roman state requires acts of public service such as holding local offices. Decurions are held responsible for preserving public order, ensuring for entertainment, and maintaining religious rituals.
Being a decurion can be a significant personal expense, and being exempt from serving as a decurion creates significant benefits for Christians. It is, in effect, a reverse tax on pagans: not only do pagans have to serve as decurions while Christians do not, but they have to do so more often because without Christians there are fewer people eligible for the office.