That he wrote also to the King of Persia, who had sent him an Embassy, on Behalf of the Christiansin his Realm
The king of the Persians also having testified a desire to form an alliance with Constantine, by sending an embassy and presents as assurances of peace and friendship, the emperor, in negotiating this treaty, far surpassed the monarch who had first done him honor, in the magnificence with which he acknowledged his gifts. Having heard, too, that there were many churches of God in Persia, and that large numbers there were gathered into the fold of Christ, full of joy at this intelligence, he resolved to extend his anxiety for the general welfare to that country also, as one whose aim it was to care for all alike in every nation.
Source: Life of Constantine (New Advent)