Small sins, according to Screwtape

You will say that these are very small sins; and doubtless, like all young tempters, you are anxious to be able to report great wickedness. But do remember, 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 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 milestones, without signposts.

Screwtape, to his nephew Wormwood.
(C.S. Lewis, The Screwtape Letters)