Augustine (City of God, Book 21, Chapter 16) explains that our hearts must be purified through faith.
And if vices have not gathered strength, by habitual victory they are more easily overcome and subdued; but if they have been used to conquer and rule, it is only with difficulty and labor they are mastered. And indeed this victory cannot be sincerely and truly gained but by delighting in true righteousness, and it is faith in Christ that gives this. For if the law be present with its command, and the Spirit be absent with His help, the presence of the prohibition serves only to increase the desire to sin, and adds the guilt of transgression. Sometimes, indeed, patent vices are overcome by other and hidden vices, which are reckoned virtues, though pride and a kind of ruinous self-sufficiency are their informing principles. Accordingly vices are then only to be considered overcome when they are conquered by the love of God, which God Himself alone gives, and which He gives only through the Mediator between God and men, the man Christ Jesus, who became a partaker of our mortality that He might make us partakers of His divinity.