Warhammer 40,000: Inquisitor - Martyr

When your Faith meter fills completely, it grows angel wings and prompts you to enter Embodied Saint form by pressing L3. Do this 100 times for the achievement. My guide for Faith generated has some good tips for filling the meter, but the long story short is to use the default Sacred Rite for it as it constantly generates Faith and increases the amount by 0.01% per enemy killed.