Regarding Sacred Fire, its usefulness depends on the type of cleric you're building.
Note that the main selling points of Sacred Fire are it's ability to hit more than one target, something that clerics sorely lack; the extra damage to undead is a nice bonus too. In general, pausing to cast Sacred Fire against single targets results in a dps loss as compared to continuous melee attacks. This doesn't apply to undead targets; against undead, fire away.
Cainus' cleric build is that of a warrior guild cleric. By joining the warrior guild, clerics get to enjoy cheaper HP potions and access to the Heavy Hand and Duel skills. This boosts their single target melee dps high enough that Sacred Fire usually has to hit 3 or more targets to out damage their melee attack dps. Also, the usage of duel discourages interruptions to their melee attacks, a disincentive to use sacred fire. IMO, the only mana skill they truly benefit from is Arbenos' Might, since it doesn't compete with, but buffs melee dps. Since they lack access to mana potions from a vendor, the limited mana is probably better saved for more Arbenos' Might anyway. For warrior guild clerics, Sacred Fire is really quite optional.
If you choose to instead join The Three, Sacred Fire might prove a little more useful. Lacking the melee dps of their warrior guild counterparts, Sacred Fire usually only needs to hit at least 2 targets to outdamage their melee dps; you'd find many more opportunities to use Sacred Fire. Also, mana potions are available for sale to clerics of The Three, so you'd have no problems sustaining a heavy usage of the skill. While not a necessity, Sacred Fire is certainly more useful to clerics of The Three. Still optional though.
Whether or not to get Sacred Fire boils down to one question. Do you want to have a multi-target attack available to fall back on?