Haven't started Demon's Winter for real yet, but I did manage to get a character to 255 SP (probably the maximum?) through thoughts and prayers. Well, maybe not thoughts, but definitely prayers.
One oddity: Outside of battle, you can only pray once per day (per character with appropriate skill?). In battle, this limit does not apply. Therefore, if worshiping a god of magic, which is the one god type you can pray to both in and out of battle, you can keep trying in battle even if your prayer isn't answered outside of battle.
If a prayer to this god is answered, you get a lot of SP (over 100, and sometimes over 200). If you get the prayer answered twice on the same day, and target the same character each time, you could easily hit the 255 cap.
(Note that the chance of a prayer being answered is low and drops further with each success, and resetting that chance requires finding a suitable church/temple and paying money.)