Of his care for the Scythians and his zealagainst the Marcionists
On learning that some of the Nomads encamped along the Danube were thirsty for salvation, but had none to bring them the stream, John sought out men who were filled with a love of labour like that which had distinguished the apostles, and gave them charge of the work. I have myself seen a letter written by him to Leontius, bishop of Ancyra, in which he described the conversion of the Scythians, and begged that fit men for their instruction might be sent.
On hearing that in our district some men were infected with the plague of Marcion he wrote to the then bishop charging him to drive out the plague, and proffering him the aid of the imperial edicts. I have said enough to show how, to use the words of the divine apostle, he carried in his heart “the care of all the churches.”
His boldness may also be learned from other sources.
Source: Ecclesiastical History (New Advent)