1 Since the Lord warns us, saying, “You are the salt of the earth,” and since He bids us to be simple to harmlessness, and yet with our simplicity to be prudent, what else, beloved brethren, befits us, than to use foresight and watching with an anxious heart, both to perceive and to beware of the wiles of the crafty foe, that we, who have put on Christ the wisdom of God the Father, may not seem to be wanting in wisdom in the matter of providing for our salvation? For it is not persecution alone that is to be feared; nor those things which advance by open attack to overwhelm and cast down the servants of God.
Caution is more easy where danger is manifest, and the mind is prepared beforehand for the contest when the adversary avows himself. The enemy is more to be feared and to be guarded against, when he creeps on us secretly; when, deceiving by the appearance of peace, he steals forward by hidden approaches, whence also he has received the name of the Serpent. That is always his subtlety; that is his dark and stealthy artifice for circumventing man. Thus from (he very beginning of the world he deceived; and flattering with lying words, he misled inexperienced souls by an incautious credulity.
Thus he endeavoured to tempt the Lord Himself: he secretly approached Him, as if he would creep on Him again, and deceive; yet he was understood, and beaten back, and therefore prostrated, because he was recognised and detected.
Source: The Treatises of Cyprian (New Advent)