When this spells is cast, the priest effectively draws on the strength of her/his god and lends it to the target creature, in effect creating a champion. The target gains a bonus to her/his THAC0 at a rate of 1 for every 3 levels of the caster. So a 9th level priest would confer a THAC0 bonus of 3 to the target, and so on. Also the target's Strength is set to 18/00 for the duration of the spell and receives all the bonuses to hit and damage rolls that this Strength confers. Note also that if the target's strength is above 18/00, it will actually be reduced to this value.
The drawback to this is that the priest must concentrate on the connection between the target and her/his god for the duration of the spell, hence losing the ability to cast any spells during this time. The effect lasts for 3 rounds for every level of the caster or until dispelled.