After the finale and Spike’s sacrifice to save the world, he immediately rematerializes as the Scoobies are about to take off. They find out at some point from Whistler that Spike was returned because his selfless sacrifice redeemed his soul. The PTB was anticipating Angel making that sacrifice, which would have been more straightforward because his act would have been tainted by his desire for redemption—therefore, not completely selfless. Since Spike had no expectations for reward, the act earned his redemption. 

Now, although he’s still a vampire and needs blood, he’s immune to sunlight, holy water, crosses, and has a reflection.