Both of the scriptures in the book of Mosiah which teach of the redemption of those who were ignorant the laws of God are followed by warnings to those who have the law given to them. Jacob does the same thing. The Lord is merciful with the ignorant because they don’t know better. For those who have the law given, obedience is the best option. For of him unto whom much is given much is required; and he who sins against the greater light shall receive the greater condemnation (DC 82:3). Mosiah 3:12 states, But wo, wo unto him who knoweth that he rebelleth against God! When Abinidi was teaching the wicked king Noah and his priests, he warned them against their sinful state, because they had the law given to them. Their sin was particularly heinous because of their willful rebellion against God:
But behold, and fear, and tremble before God, for ye ought to tremble; for the Lord redeemeth none such that rebel against him and die in their sins; yea, even all those that have perished in their sins ever since the world began, that have wilfully rebelled against God, that have known the commandments of God, and would not keep them; these are they that have no part in the first resurrection.
Therefore ought ye not to tremble? For salvation cometh to none such; for the Lord hath redeemed none such; yea, neither can the Lord redeem such; for he cannot deny himself; for he cannot deny justice when it has its claim (Mos 15:27-8).
“The Atonement Satisfieth the Demands of Justice Upon All Those Who Have Not the Law”
The atonement has power to save those in a state of innocence. In particular, this applies to two main groups, children and those who have not the law given to them. This principle is taught in the law of sacrifice as found in the law of Moses, If a soul shall sin through ignorance against any of the commandments of the Lord….[the priest] shall do with the bullock as he did with the bullock for a sin offering…the priest shall make an atonement for them, and it shall be forgiven them (Lev 4:2,20).
The Book of Mormon teaches this principle in several different locations. King Benjamin’s sermon includes the following, 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 (Mosiah 3:11). Mormon teaches that both children and those without the law will be saved, For behold that all little children are alive in Christ, and also all they that are without the law, for the power of redemption cometh on all them that have no law (Moroni 8:22).
The scriptures go so far as to say that those without the law will come forth in the first resurrection, And then shall the heathen nations be redeemed, and they that knew no law shall have part in the first resurrection; and it shall be tolerable for them (DC 45:54). They will come forth in the first resurrection after those who are “the first fruits” (or those who come forth in the morning of the first resurrection). For the most part, they will inherit the terrestial kingdom and will be resurrected as “those who are Christ’s at his coming” (or those who come forth in the afternoon of the first resurrection). See DC 88:98-9 and DC 76:71-73. If you are still confused by this doctrine, read Mosiah 15:24-25, Heb 5:2, DC 137:7, and Mormon Doctrine, p. 640.