Potential soulfire rewrite to take into account what we've learned since:
SOULFIRE [-4]Description: You have access to the fires of creation, the same power used by angels.
Sponsor: Heaven, usually via one of the Archangels
Agenda: Exact details are unclear, but seems to be related to creation, the promotion of mortal free will, and perhaps opposing dark and evil forces. May also be connected to faith.
Evocation: Soulfire evocations tend to produce intense, blue-white light. If used as an element, it acts as fire, with a special focus on fire's purifying aspects.
Thaumaturgy: Soulfire thaumaturgy can produce effects related to creation – conjuration, illusions, wards. It can also banish hostile or dark magic.
Evothaum: Soulfire can produce creation-related effects (conjuration, illusions, wards) with the speed of evocation.
Extra Benefits: Soulfire is especially potent when used to create. Conjurations and illusions created with soulfire (including evothaum) gain a +1 complexity and control bonus.
I think that covers everything we've seen Harry use it for, with the possible exception of (CD spoiler)
breaking the will-block of Mother Winter, and even that falls under "banish hostile or dark magic", it's just way more powerful than Harry should be able to do.
But I think that's an effect of Starborn + Soulfire synergy, likely plus stuff he learned from being dead, not a base effect of the Soulfire power alone.