Around Easter in 356 (only months after having been banished for defending the orthodox formulation of the Trinity against heresy), Athanasius wrote to the Bishops of his province in Egypt to warn them against a creed that would soon be circulated that taught the false doctrine that Athanasius rejected as being contrary to the true orthodox faith.
Schaff says that this was probably the Sirmian Creed of 351.  Much like certain teachers of our own day who have gotten much press in the blogosphere of late, the Sirmian Creed did not formally embrace the heretical teachings of Arianism, but at the same time fell short of affirming what the church had affirmed as orthodoxy at the Council of Nicea.
The Lord said, that ‘there shall arise false Christs and false prophets, so that they shall deceive many’ (Matt. 24:24). Accordingly the devil has come, speaking by each and saying, ‘I am Christ, and the truth is with me;’ and he has made them, one and all, to be liars like himself [cf. John 8:44]. And strange it is, that while all heresies are at variance with one another concerning the mischievous inventions which each has framed, they are united together only by the common purpose of lying. For they have one and the same father that has sown in them all the seeds, of falsehood.
Wherefore the faithful Christian and true disciple of the Gospel, having grace to discern spiritual things, and having built the house of his faith upon a rock, stands continually firm and secure from their deceits.
But the simple person, as I said before, that is not thoroughly grounded in knowledge, such an one, considering only the words that are spoken and not perceiving their meaning, is immediately drawn away by their wiles.
Wherefore it is good and needful for us to pray that we may receive the gift of discerning spirits [cf. 1 John 4:1], so that every one may know, according to the precept of John, whom he ought to reject, and whom to receive as friends and of the same faith.
Athanasius, “To the Bishops of Egypt,” 1:4 from A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume IV: St. Athanasius: Select Works and Letters, ed. Philip Schaff and Henry Wace (New York: Christian Literature Company, 1892), 225.
Note that the emphasis is mine.
 A Select Library of the Nicene and Post-Nicene Fathers of the Christian Church, Second Series, Volume IV: St. Athanasius: Select Works and Letters, ed. Philip Schaff and Henry Wace (New York: Christian Literature Company, 1892), 222.