If your life total is less than 1 but you've somehow avoided losing the game, Fortune Thief's effect won't apply at all if you're dealt damage.

Fortune Thief's effect applies only if your life total is being reduced by damage. Other effects or costs (such as losing or paying life) can reduce your life total below 1 as normal.

Fortune Thief's effect is not a prevention effect. It stops unpreventable damage from reducing your life total below 1.

Fortune Thief doesn't change how much damage is dealt; it just changes how much life that damage makes you lose. Abilities such as lifelink will see the full amount of damage being dealt.

Fortune Thief won't prevent you from losing the game if your life total becomes 0 or less or if some other effect causes you to lose the game.