Deific weapon, as written, does not give the paladin any benefit if their deity is LG or NG, since all those deities' favored weapons are martial. Some of the LN or N deities' favored weapons are simple. But that means this rule is only beneficial to paladins of a select few deities.
I would change the wording to the Deific Weapon rule to say the paladin is always trained in his deity's favored weapon, and upon reaching 2nd level treats the damage of his deity's favored weapon as 1 die size higher. If the paladin must perform an atonement he loses the higher damage ability until he gains another level after completing the atonement requirements.