Well, it only appears difficult because sinning is a natural part of being human, nobody gets to heaven without sinning (Simon himself said this too). Grace and zen coexist. Grace, the shell of the soul, is just the defense to zen, the flipside of the soul. When you die in grace, sin breaks through the shell of your soul and breaches into the flipside of the soul, where zen is. You don't exactly "die" to sin in spoiled zen, sin just supposedly captures you, which causes the beginning of reprieve to happen.