3 We should make a difference, dearest brother, between those who either have apostatized, and, having returned to the world which they have renounced, are living heathenish lives, or, having become deserters to the heretics, are daily taking up parricidal arms against the Church; and those who do not depart from the Church's threshold, and, constantly and sorrowfully imploring divine and paternal consolation, profess that they are now prepared for the battle, and ready to stand and fight bravely for the name of their Lord and for their own salvation.
In these times we grant peace, not to those who sleep, but to those who watch. We grant peace, not amid indulgences, but amid arms. We grant peace, not for rest, but for the field of battle. If, according to what we hear, and desire, and believe of them, they shall stand bravely, and shall overthrow the adversary with us in the encounter, we shall not repent of having granted peace to men so brave. Yea, it is the great honour and glory of our episcopate to have granted peace to martyrs, so that we, as priests, who daily celebrate the sacrifices of God, may prepare offerings and victims for God.
But if— which may the Lord avert from our brethren— any one of the lapsed should deceive, seeking peace by guile, and at the time of the impending struggle receiving peace without any purpose of doing battle, he betrays and deceives himself, hiding one thing in his heart and pronouncing another with his voice. We, so far as it is allowed to us to see and to judge, look upon the face of each one; we are not able to scrutinize the heart and to inspect the mind. Concerning these the Discerner and Searcher of hidden things judges, and He will quickly come and judge of the secrets and hidden things of the heart.
But the evil ought not to stand in the way of the good, but rather the evil ought to be assisted by the good. Neither is peace, therefore, to be denied to those who are about to endure martyrdom, because there are some who will refuse it, since for this purpose peace should be granted to all who are about to enter upon the warfare, that through our ignorance he may not be the first one to be passed over, who in the struggle is to be crowned.
Source: The Epistles of Cyprian (New Advent)