Oh, I love this.
5 For Moses writes about the righteousness which is of the law, “The man who does those things shall live by them.” 6 But the righteousness of faith speaks in this way, “Do not say in your heart, ‘Who will ascend into heaven?’ ” (that is, to bring Christ down from above) 7 or, “ ‘Who will descend into the abyss?’ ” (that is, to bring Christ up from the dead). 8 But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith which we preach):
Moses' original illustration was in fact, that the Law he had given was not beyond their reach, in the sense of finding out what it commands. That they didn't need to climb the highest mountain to reach some guru, or somehow sail over the farthest seas to some remote kingdom.
Paul repurposes this illustration to make a vital point about self-attainment and the Work of Christ. Christ fulfilled the Law for us both positively and negatively, that is, he positively met the demands for righteous living, but he also met the Law's strict and unyielding demand for a punishment for every infraction.
When we understand a righteousness by faith as a gift instead of earning it through Law-keeping, we see that no amount of positive deeds can cause us to be good enough to ascend to heaven; and no amount of negative penance or sorrow, can be holy enough to truly erase the evils we have done.
Instead of our meritorious works, Christ was the one who in the preceding undeserved grace of God took the initiative to come down from heaven and live a perfect life on our behalf, and then he went on to "descend to the abyss," the dark punishments of sin and place of the dead, to defeat death and hell on our behalf.
Now this Word of Faith, that all we have to do is reach out and receive a gift, is the far better good news brought to us than the constant burden of trying to meet all the Law's demands for holiness and righteous living that weigh us down with obligation and guilt and puff us up with pride.
Religion says "DO.'
God says "DONE."