Mormons haven't thoroughly explained their version of the atonement. Basically, it was so that each human would be resurrected even those who will spend eternity with the devil in outer darkness. The other part of the atonement pays for the sins of those who obey the commandments (including all Mormon laws and ordinances). The atonement provides help to those who "do all they can do."
Doctrine and Covenants 130
20 There is a law, irrevocably decreed in heaven before the foundations of this world, upon which all blessings are predicated—
21 And when we obtain any blessing from God, it is by obedience to that law upon which it is predicated.
How Can We Access the Savior’s Grace?
When we need guidance or support or strength, God can help us. But it depends on us too. We have to seek His help and be worthy of it.
As Elder Richard G. Scott of the Quorum of the Twelve Apostles has said,“When we obey the commandments of the Lord and serve His children unselfishly, the natural consequence is power from God—power to do more than we can do by ourselves. Our insights, our talents, our abilities are expanded because we receive strength and power from the Lord.”
Joshua J. Perkey, Grace and the Atonement of Jesus Christ
Doctrine and Covenants 25
15 Keep my commandments continually, and a crown of righteousness thou shalt receive. And except thou do this, where I am you cannot come.
The lowest kingdom of glory:
Doctrine and Covenants 76
98 And the glory of the telestial is one, even as the glory of the stars is one; for as one star differs from another star in glory, even so differs one from another in glory in the telestial world;
99 For these are they who are of Paul, and of Apollos, and of Cephas.
100 These are they who say they are some of one and some of another—some of Christ and some of John, and some of Moses, and some of Elias, and some of Esaias, and some of Isaiah, and some of Enoch;
101 But received not the gospel, neither the testimony of Jesus, neither the prophets, neither the everlasting covenant.
102 Last of all, these all are they who will not be gathered with the saints, to be caught up unto the church of the Firstborn, and received into the cloud.*
103 These are they who are liars, and sorcerers, and adulterers, and whoremongers, and whosoever loves and makes a lie.
104 These are they who suffer the wrath of God on earth.
105 These are they who suffer the vengeance of eternal fire.
106 These are they who are cast down to hell and suffer the wrath of Almighty God, until the fulness of times, when Christ shall have subdued all enemies under his feet, and shall have perfected his work;
107 When he shall deliver up the kingdom, and present it unto the Father, spotless, saying: I have overcome and have trodden the wine-press alone, even the wine-press of the fierceness of the wrath of Almighty God.
108 Then shall he be crowned with the crown of his glory, to sit on the throne of his power to reign forever and ever.
109 But behold, and lo, we saw the glory and the inhabitants of the telestial world, that they were as innumerable as the stars in the firmament of heaven, or as the sand upon the seashore;
110 And heard the voice of the Lord saying: These all shall bow the knee, and every tongue shall confess to him who sits upon the throne forever and ever;
111 For they shall be judged according to their works, and every man shall receive according to his own works, his own dominion, in the mansions which are prepared;
112 And they shall be servants of the Most High; but where God and Christ dwell they cannot come, worlds without end.
* I'll start a thread about the church of the Firstborn. Those in the telestial kingdom are not the only ones excluded from it.
