"do all we can do." And I disagree with the common interpretation of 2 Nephi 25:23 that suggests we must "do all we can do."
It is doctrinally true that only Christ paid for our sins. I do not have the ability to redeem myself from sin. Also, I he did not pay for some of my sins, nor did he pay for a certain percentage of all my sins, waiting on me to pay the rest. Instead, he already paid the price for 100% of all my sins. This is doctrine.
Does that mean salvation is already guaranteed? Of course not. It is still conditional on some action on our part, even if that action does not require that we "do all we can do." After all, is there anyone who has done everything possible to be obedient? That's a standard we would all fall short of.
In sum, Christ's Atonement is infinite in that he paid the full price for all our sins. But He does not force redemption on us, and leaves it up to us whether or not to accept his grace. This is where our works come in, because we show our acceptance of His grace by being obedient to His commandments.