This article studies the two earliest witnesses to the Apostles' Creed which have often been regarded with suspicion because they were written by "heretical" bishop-theologians Marcellus of Ancyra and Priscillian of Avila. Assessing the thought of Marcellus and Priscillian in the light of their authentic treatises, it is contended that their respective understandings of Trinitarian theology cannot be identified with modalist monarchianism. Consequently, their creeds should not be regarded as smoke screen for their allegedly deviant doctrines. Rather, these should be regarded as the first extant evidence for the declaratory Apostles' Creed. This article studies the two earliest witnesses to the Apostles' Creed which have often been regarded with suspicion because they were written by "heretical" bishop-theologians Marcellus of Ancyra and Priscillian of Avila. Assessing the thought of Marcellus and Priscillian in the light of their authentic treatises, it is contended that their respective understandings of Trinitarian theology cannot be identified with modalist monarchianism. Consequently, their creeds should not be regarded as smoke screen for their allegedly deviant doctrines. Rather, these should be regarded as the first extant evidence for the declaratory Apostles' Creed.