Elder McConkie regarding those who transgress after having Calling and Election made sure:



[11] What if those whose calling and election has been made sure thereafter commit grievous sins? Suppose they backslide and walk in the ways of wickedness? Or fight the truth and rebel against God--what then?
That all men commit sin, before and after baptism, and for that matter, before and after their calling and election is made sure, is self-evident. There has been only one Sinless One--the Lord Jesus who was God's own Son.
Thus in the revelation announcing the setting up of the restored church in this day, the Lord says: "There is a possibility that man may fall from grace and depart from the living God; Therefore let the church take heed and pray always, lest they fall into temptation; Yea, and even let those who are sanctified take heed also." (D&C 20:32-34.)
The prophets and apostles from Adam and Enoch down, and all men, whether cleansed and sanctified from sin or not, are yet subject to and do in fact commit sin. This is the case even after men have seen the visions of eternity and been sealed by that Holy Spirit of Promise which makes their calling and election sure. Since these chosen ones have the sure promise of eternal life, and since "no unclean thing can enter into" the Father's "kingdom" (3 Ne 27:19), "or dwell in his presence" (Moses 6:57), what of sins committed after being sealed up into eternal life?
Obviously the laws of repentance still apply, and the more enlightened a person is, the more he seeks the gift of repentance, and the harder he strives to free himself from sin as often as he falls short of the divine will and becomes subject in any degree to the Master of Sin who is Lucifer. It follows that the sins of the godfearing and the righteous are continually remitted because they repent and seek the Lord anew every day and every hour.
And as a matter of fact, the added blessing of having one's calling and election made sure is itself an encouragement to avoid sin and a hedge against its further commission. By that long course of obedience and trial which enabled them to gain so great a blessing the sanctified saints have charted a course and developed a pattern of living which avoids sin and encourages righteousness. Thus the Lord said: "I give unto you Hyrum Smith to be a patriarch unto you, to hold the sealing blessings of my church, even the Holy Spirit of promise, whereby ye are sealed up unto the day of redemption, that ye may not fall notwithstanding the hour of temptation that may come upon you." (D&C 124:124.)

But suppose such persons become disaffected and the spirit of repentance leaves them--which is a seldom and almost unheard of eventuality--still, what then? The answer is--and the revelations and teachings of the Prophet Joseph Smith so recite!--they must then pay the penalty of their own sins, for the blood of Christ will not cleanse them. Or if they commit murder or adultery, they lose their promised inheritance because these sins are exempt from the sealing promises. Or if they commit the unpardonable sin, they become sons of perdition.

 Bruce R. McConkie, Doctrinal New Testament Commentary, 3:325-353.