"If a man therefore purge himself from these, he shall be a vessel unto honour, sanctified, and meet for the master’s use, and prepared unto every good work."(New Testament, 2 Timothy, 2:21)
True Christian Doctrine is repentance focused. Yet, many churches tend to be more focused on grace alone. Many false doctrines teach that if we except Christ we are forever saved and repentance is good but ultimately doesn't affect the believer's salvation because Paul taught we are saved by grace through faith alone.
"For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast. (New Testament, Ephesians 2:8-9)
But here is the critical point why repentance and righteousness matter. Repentance through the grace and power of Christ makes use useful in the service of Christ not only here on Earth but also in the eternities.
Can you imagine accepting Christ and continuing in sin? Can you imagine being saved in heaven without being useful because we refused to repent and remain in our sins?
When we are resurrected and are brought to stand before God, the Book of Mormon teaches that when Christ redeems us we will be resurrected but restored to the same attitude we are in now. If we are useful now we will be useful then. But if, because of sin, we are useless now, we will continue to be useless then and for eternity.
In the Book of Mormon an ancient American prophet Alma (the younger) is speaking to his son Corianton. Corianton was called to be a missionary and preach the word of Christ to the people. However, Corianton misunderstood the doctrine of grace and repentance, justice and mercy; and while in the service of God excussed himself in committing sexual sin. Because of his bad behavior and example, Alma confesses that Corianton ceased to be useful and the people would no longer believe the word of Christ. Alma proceeds to correct Corianton's mistaken understanding of grace which led to his sinful behavior.
"Do not suppose, because it has been spoken concerning restoration, that ye shall be restored from sin to happiness. Behold, I say unto you, wickedness never was happiness...
O, my son, this is not the case; but the meaning of the word restoration is to bring back again evil for evil, or carnal for carnal, or devilish for devilish—good for that which is good; righteous for that which is righteous; just for that which is just; merciful for that which is merciful....
For that which ye do send out shall return unto you again, and be restored; therefore, the word restoration more fully condemneth the sinner, and justifieth him not at all. (Book of Mormon, Alma 41:10,13,15)
This teaching is found in the Book of Mormon precisely because God knew many in our day would have the same misunderstanding.