Nobody burns a heretic, a Xenos or a random passer-by more enthusiastically than a priest of the God Emperor. Accordingly, the Sanctifiers bring all sorts of fire-based weaponry, from the humble (though still holy) hand flamer to the extra-holy fire manifested by the Miraculist. These are mostly guns but Missionaries can bring a Brazier of Holy Fire, which works in melee too. All of them have the Blaze rule.
If one of these weapons retains a crit against an enemy they catch fire. Whenever that operative is activated they take D3 damage. Then they can choose to act as normal and have the fire go out on a 3+ or they can sacrifice an APL to put it out automatically. Otherwise they keep burning.