Behind the boundaries, east of the Euphrates River, the Sassanid governors of the Persian Empire had commonly tolerated their Christians. With the orders of toleration in the Roman Empire, Christians in Persia would now be considered as allies of Persia's ancient foe, and were thus oppressed. A letter purportedly from Constantine to Shapur II of Persia and supposed to have been written in c. 324 inspired Shapur to protect the Christians in his lands. Shapur II, with that, wrote to his generals.
You will hold Simon, head of the Christians. You will sustain him until he marks this document and accepts to collect for us a double tax and double testimonial from the Christians for we Gods have all the tests of war and they have nothing but repose and delight. They dwell our dominion and accord with Caesar, our foe. [Freya Stark, Rome on the Euphrates, 1967, page 375].
However the document named the (Gift of Constantine) was proved a counterfeit in the 15th century, when the tales of Constantine's changeover were long-established "Facts" it was for centuries thought to document the conversion of the Roman Empire and apologize Papal primacy and working powers.