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.

Calling and Election

This, then, without amplifying, without saying more, is the doctrine that is involved. It is a true doctrine; it is a sound doctrine. What does it mean to us? It is not something that is beyond us; it is not outside the realm of our capabilities. The Lord would not offer baptism to a people and say, "Be baptized and you can be saved in my kingdom," unless he knew that the people had it within their power to enter into baptism and to do the things that will result in the promised salvation. And the Lord would not offer to a people celestial marriage and say to them, "Now enter this order of matrimony and keep the covenant and you will have eternal life," unless he knew that the people had the talent and the spiritual capacity and ability to gain the promised reward. And the same thing applies to making our calling and election sure. The Lord would not say to us, "Make your calling and election sure," unless he knew it was within our spiritual potential -- unless we had the capacity and ability to press forward in steadfastness and devotion until that desired eventuality occurred.
Now, I am suggesting that this is within the realm of possibility and of probability for the members of the Church today. Many in this dispensation have obtained such a status. This ought to cause us to make a firm and unshakable determination to gain the blessings and benefits of baptism; to gain the blessings and benefits of celestial marriage; and in each instance those blessings accrue on condition that we keep the covenants that we make in connection with those holy ordinances. And then we ought to press forward with a steadfastness in Christ, keeping the commandments and living by every word that proceedeth forth from the mouth of God, so that our calling and election shall be made sure. In one manner of speaking, this is the most glorious doctrine that there is in the Church. It is the doctrine that assures people of eternal reward in the kingdom of God. By the grace and glory and goodness of a merciful Father, he has restored in this, our day, the knowledge, the doctrine, the keys, the priesthood, the powers -- everything that is needed to seal men up unto eternal life. Our objective, our goal, our aim, our determination in this life is to press forward with that steadfastness and devotion which will give us complete peace and satisfaction and joy in this life and assure us of eternal life in the world to come.  McConkie BYU Devotional 1969