Finding out one of your children is not making the right choices and feeling like a complete failure as a parent. There is no worse pain. It feels like someone shoved a machete into your chest and tweaked out your heart. Then, you pick up the pieces and move on, because you love your child no matter what.
I also, as a Christian, stand on the truth of God's word. He has promised me that if I raise a child up in the way he should go, that when he is old he will not depart from it.
Losing people you love is a similar kind of pain. There is a grieving process.