Eusebius: "Guard against the heresies"

“WHILE [Ignatius of Antioch] was making the journey through Asia under the strictest military guard, he strengthened the diocese in each city where he stayed by spoken sermons and exhortations, and he especially exhorted them above all to be on their guard against the heresies which then for the first time were prevalent and he urged them to hold fast to the tradition of the apostles to which they thought it necessary, for security’s sake, to give form by written testimony.”

~Eusebius of Caesarea:  Ecclesiastical History, Bk. 3, Chap. 36.