Bonosus was an Illyrian bishop, probably of Serdica, who lived at the end of the 4th century, and followed Helvidius in denying the perpetual virginity of Mary.
The letter presented here is the second of the letters sent to the Illyrian bishops, dealing with the issue of the presbyters and deacons ordained by Bonosus before his condemnation. It was traditionally dated to ca AD 409/410, but Dunn 2013 argues that it must have been composed between AD 412 and 414, when Letter 17  was written.
This letter was caused by the mission of Germanius and Lupentius who came to Ravenna to complain to Innocent about the reordination of Rusticius, required by Marcian as the condition to accept other clerics ordained by Bonosus. "The fact that Innocent involved himself in a matter about presbyters was contrary to, or an expansion of, canon 17 of the Synod of Serdica in 343, that limited the right of appeal of lower clergy against their bishop to neigbouring bishops only" (Dunn 2013: 333).
Rusticius and other clerics in question seem to act as the pastors of some Churches in their own right ("ecclesias sibi creditas").