Why can't Satan or the world touch a believer?
Because the believer's new life is hidden with Christ in God, an act of the Almighty hides them beyond reach. Satan cannot get at them there; the world cannot get at them there; no one can touch them. This hidden position is their assurance and promise.
Les teaches that Paul says believers are dead to those things, and the new eternal life imputed to us is hid. If the God who created the universe hides something, no one but Him will ever find it. He hides us with Christ and in God. Les compares this to a walnut: the meat lies deep inside the outer shell, the tough shell, and the crevices, where no one but God can find it. That is where believers are. Satan cannot get at us there, the world cannot get at us there, and no one can touch us, because we are hid with Christ in God. Les calls this assurance and a promise, even while noting we do not feel an outer covering and remain vulnerable.