The so-called Endless Peace treaty, signed between Rome and Persia in 532, contained several provisions that regulated issues of population transfer. According to the famous evidence of Agathias of Myrina, in the treaty there was also a clause guaranteeing safety from persecution and the tolerance of religious beliefs in the territory of the Roman Empire for the seven Neoplatonic philosophers returning from their Persian emigration. The present article proposes a re-evaluation of the clause mentioned by Agathias by extracting parallel information from an East-Syriac hagiographical source: an anonymous account of martyrdom of the high-profile Persian Christian convert Mar Grigor. The study deconstructs Agathias' evidence regarding the circumstances of the philosophers' emigration and return, and examines the available set of “conventional” sources on how the Endless Peace treaty regulated the status of different categories of displaced people. The investigation proceeds with an analysis of the Martyrdom of Mar Grigor , arguing for the importance of the East-Syriac hagiographical account for a comprehensive reconstruction of the conditions of the Endless Peace agreement. Assessing information provided by the Martyrdom of Mar Grigor and other available data, the author reveals the high relevance of the East-Syriac evidence for the discussion of the so-called clause of protection. The scope of the article is to demonstrate, for the first time in historiography, that the clause, included in the treaty to protect the seven Hellenic philosophers upon their return to the Christian Roman Empire, was not unilateral. It is suggested that the same diplomatic agreement contained a similar promise of safe conduct for the Christian Persian general, Pīrān-Gušnasp / Mar Grigor, coming back from Roman captivity to Zoroastrian Persia.