when he wishes to deprive you of the courage that the Lord bestows upon you, and when he tries to render you weak and timorous by means of his snares and deceits, do not merely venture to say that you are desirous of serving Our Lord, when you ought to proclaim and profess without fear that you are His servant, and that you would rather die than desert His service.
If he puts before me the justice of God, I reply with His mercy; if he hints at mercy, I answer with His justice. So we must act if we would avoid trouble, that the deceiver may himself be deceived, applying to ourselves the teaching of Holy Scripture which says: "Beware that thou be not so humble that in excessive humility thou be led into folly." "Coming to the second matter, as the enemy has placed in us a certain fear under the appearance of humility, which is false, and so suggests that we ought not to speak even of good, holy, and profitable things, so he brings forward another and worse fear, which is, whether we are separated from Our Lord, cut off from Him and outcast, and this in great measure by reason of our past lives. For just as the first fear prepares the way for the enemy's victory, so he finds it easier to tempt us when we are subject to the second. To illustrate this in some way I will mention another of the enemy's devices. If he finds a person with an elastic conscience, who passes over sin without consideration, he does all in his power to make venial sin seem nothing, and mortal sin, even very serious mortal sin, of no account; so that he turns to his purpose the defect he finds in us, that of a too elastic conscience.
If in another he discovers a conscience over tender— a tender conscience, be it noticed, is no fault,—and if he sees that such a one will have nothing to do with mortal sin, nor even with venial sin so far as is possible—for it is not in our power to avoid all—,
, and that he even tries to cast off every slight semblance of sin in the shape of imperfection or defect, then the enemy makes an effort to confuse so good a conscience, suggesting sin where there is none, and defect where there is even perfection, anything to be able to disturb and afflict us; and in many instances, where he cannot induce a soul to sin, and has no hope of ever bringing it about, at least he endeavours to torment.
Link (here) to Letters and Instruction of St. Ignatius Loyola