The tempter is foiled in attacks upon those who have learned these tactics
And so those teachers, who have instructed all the Church's sons by their examples and their traditions, began the rudiments of the Christian warfare with holy fasts, that, having to fight against spiritual wickednesses, they might take the armour of abstinence, wherewith to slay the incentives to vice. For invisible foes and incorporeal enemies will have no strength against us, if we be not entangled in any lusts of the flesh. The desire to hurt us is indeed ever active in the tempter, but he will be disarmed and powerless, if he find no vantage ground within us from which to attack us. But who, encompassed with this frail flesh, and placed in this body of death, even one who has made much decided progress, can be so sure of his safety now, as to believe himself free from the peril of all allurements? Although Divine Grace gives daily victory to His saints, yet He does not remove the occasion for fighting, because this very fact is part of our Protector's Mercy, Who has always designed that something should remain for our ever-changing nature to win, lest it should boast itself on the ending of the battle.
Source: Sermons (New Advent)