Thomas Brooks warns against the deceptive nature of 'little sins,' emphasizing that even minor transgressions can lead to significant spiritual danger. He illustrates how small sins, often overlooked, can infiltrate the soul and grow into greater issues, ultimately leading to spiritual death. Brooks uses vivid analogies, such as a little leaven affecting the whole lump and a small hole sinking a ship, to convey the seriousness of seemingly insignificant sins. He urges believers to be vigilant and recognize that neglecting these 'little sins' can have dire consequences for their souls.