The description in Mosiah 3 sounds as if those who don't have the full gospel have a free ticket. It says that Christ's blood atoneth for the sins of those who have died not knowing the will of God concerning them.
Mosiah 3:11 For behold, and also his blood atoneth for the sins of those who have fallen by the transgression of Adam, who have died not knowing the will of God concerning them, or who have ignorantly sinned.
This verse is true enough, but it is, as often happens, only a hint of how that actually works. It is fleshed out in D&C 128 and 138, e.g. in D&C 138 Christ teaches in the spirit world:
D&C 138:19 And there he preached to them the everlasting gospel, the doctrine of the resurrection and the redemption of mankind from the fall, and from individual sins on conditions of repentance.
So there the message was the same redemptive message we have here: "Redemption of all mankind from the fall, and from individual sins on condition of repentance." That is how Mosiah 3:11 is brought to pass.
Not that we didn't know that, but it is interesting to see how a verse that gave only a hint of its full meaning unravels into a more complete picture given additional revelation.