2 Nephi 2

Lehi said to his son, Jacob -

"...I know that thou art redeemed, because of the righteousness of thy Redeemer; for thou hast beheld that in the fulness of time he cometh to bring salvation unto men.
4 And thou hast beheld in thy youth his glory; wherefore, thou art blessed even as they unto whom he shall minister in the flesh..."  (Book of Mormon | 2 Nephi 2:3 - 4)

Even before the Savior had actually atoned for our sins, it is apparent that many received the blessings of the atonement before, even before our birth...which gives greater meaning of "the infinite atonement.  Elder Holland said -

“We could remember that even in the Grand Council of Heaven He loved us and was wonderfully strong, that we triumphed even there by the power of Christ and our faith in the blood of the Lamb” (Elder Jeffrey R. Holland, General Conference, October, 1995)

And the New Testament Student Manual reads of our pre-mortal life -

“We were given laws and agency, and commandments to have faith and repent from the wrongs that we could do there.” “… Man could and did in many instances, sin before he was born…”  (New Testament Student Manual, 1979 edition, Rel. 211–12, Institute of Religion, Page 336.)

Lehi teaches that the atonement of Jesus Christ in the plan of salvation and purposes of this world was essential in that -

"...men are instructed sufficiently that they know good from evil. And the law is given unto men. And by the law no flesh is justified; or, by the law men are cut off. Yea, by the temporal law they were cut off; and also, by the spiritual law they perish from that which is good, and become miserable forever.
6 Wherefore, redemption cometh in and through the Holy Messiah; for he is full of grace and truth."  (Book of Mormon | 2 Nephi 2:5 - 6)

Which suggests to me that without the atonement of Jesus Christ, even if we live the law of Moses perfectly we cannot be saved.  And in that all have sinned spiritually, all would suffer spiritual death.