JESUS
Without Jesus and our belief in Him, we would have NEVER gotten together and gotten married. It is through our belief in Him that we make our marriage work (or I should say He makes it work)
Hmmm...maybe that was way too broad...
My second choice would be: trust
We hide nothing from each other and we can read each other pretty well. We really are bad at lying (well I am the WORST poker player EVER! Hubby is pretty good at poker but can't lie to my face EVER! I really can tell when he is lying
He is a terrible actor!). We know we have each other's best interest at heart for everything we do.