In letter XII, Screwtape admonishes Wormwood that small sins matters:
…The only thing that matters is the extent to which you separate the man from the Enemy. It does not matter how small the sins are, provided that their cumulative effect is to edge the man away from the Light and out into the Nothing. Murder is no better than cards if cards can do the trick. Indeed, the safest road to Hell is the gradual one–the gentle slope, soft underfoot, without sudden turnings, without mile-stones, without signposts.