Conviction reduces the defense of nearby monsters, but it is almost exclusively used for it's ability to also lower the Fire, Cold and Lightning resistance of monsters as well. Together with the Necromancer'sLower Resist, they are the only two skills in the game capable of breaking an Elemental immunity, which means lowering a monster below 100% resist. All other skills, like Cold Mastery, cannot affect a monster whose immunity is 100% or greater. This makes conviction a very useful skill for both groups, and for Paladin builds utilizing Vengeance, such as a Vindicator.
Lower resist caps out at level 25 with 150% reduction, meaning no matter the point investment after that it will never get higher. Therefore you should consider how many +skills more than 5 that you will have when your character is complete, before maxing out this skill.
It is fearsome enough to behold the power of a Paladin, let alone a Paladin aglow with the aura of Conviction. This halo of righteousness demonstrates, with force, the grim determination of those who shine within its brilliance. Any who stand against the Paladin and his allies will understand the meaning of folly.
↑Diablo II Manual, page 82. Blizzard Entertainment, 2000.