Soak the raisins in the warm water for 10 minutes until plump, then drain and set aside.
Toast the pine nuts in a dry 12-inch skillet over medium heat for 2 to 3 minutes, stirring often, until golden and fragrant. Tip onto a plate.
Rinse the spinach and shake off the excess water, leaving just a little clinging to the leaves. Trim any thick stems.
Heat the olive oil in the same 12-inch skillet over medium heat. Add the garlic and cook for 30 to 45 seconds until fragrant but not browned.
Add the spinach in two or three batches, letting each batch wilt down before adding the next. This keeps the pan from overflowing and the leaves from steaming instead of sautéing.
Once all the spinach is wilted, about 4 to 5 minutes total, stir in the drained raisins, toasted pine nuts, salt, pepper, and chili flakes if using.
Cook for another 1 to 2 minutes, stirring, until any excess liquid has cooked off and the spinach looks glossy rather than watery.
Taste and adjust the salt. Serve warm, straight from the skillet.