Penetration is calculated by the requirements of the spell.

For every 2 Levels above the Spell's path Requirement you are given 1 MR Penetration.

So Banishment has 1 MR Penetration at Holy 3 and 2 MR Penetration Holy 5.

Smite Demon has 1 MR Penetration at Holy 5.

This works for all spells.