Dealing with the Devil, by Father de Caussade

You should remember all your life that one of the principal causes of the small progress made by certain good people is that the devil continually fills their souls with disquiet, perplexities, and troubles, which render them incapable of serious, gentle, and constant application to the practice of virtue. The great principle of the interior life lies in peace of the heart: it must be preserved with such care that the moment it is in danger everything else should be abandoned for its re-establishment, just as when the house is on fire, one leaves everything in order to extinguish it... This blessed peace of soul is the high road to heaven. And the reason of this is that peace and tranquility of spirit alone give the soul great strength to achieve all that God wills, while trouble and disquiet turn the soul into a weak, languishing invalid. In that state, one feels neither zest nor attraction for virtue, but, contrariwise, disgust and discouragement by which the devil never fails to profit. This is why he makes use of all his ruses to rob us of this peace on a thousand specious pretexts: at one time on pretence of examination of conscience or of sorrow for our sins, at another time on the ground that we are abusing grace and that our total lack of progress is our own fault, in short that God is about to abandon us; and by means of a hundred other dodges against which few are able to defend themselves. This is why the masters of the spiritual life give this great principle for distinguishing the true inspirations of God from those which come from the devil, namely, that the former are always gentle and peaceful and lead us to confidence and humility while the latter are agitating, unquiet, and turbulent, leading to discouragement and suspicion, or even to presumption and the following of our own will. We must, therefore, firmly reject all that does not bear this mark of peace, submission, gentleness, and confidence, the impressions as it were of God's seal; this point is of great importance for the whole of our life.

Father Jean-Pierre de Caussade, S.J., (died 1751) was a French Jesuit, a writer, and a revered spiritual director.