As a WHM, det is your going to be your main stat that you try and get as much of. Vitality, mind and piety is automatically going to be on your end game pieces so you really don't have to worry about that much.
Det increases the amount of HP you heal. The more det you have, the bigger your heals will be.
Alkimi has it right that crit is the stat that scholars build up on because it doubles their shield strength.
For WHM it should go MND>DET>SS>CRIT
White mage is all about big consistant heals.