I think that just as a sinner can commit the unpardonable sin and fall from grace and be sealed unto eternal damnation before the day of judgement, so too can a righteous person achieve a certain level of righteousness in their life to be sealed up unto eternal life before he dies and is judged. In other words, there are those who put enough time and effort that their judgement is moved up and they have their calling and election made sure unto eternal life. This also occurs with the wicked who become so wicked and commit the unpardonable sin that their damnation is sealed.
In 2 Peter 1 we also have this verse which is related to having one's calling and election made sure:
2 Peter1: 19
19
We have also a more
sure word of prophecy; whereunto ye do well that ye take heed, as unto a light that shineth in a dark place, until the day dawn, and the day
star arise in your hearts:
Joseph Smith taught the following regarding this subject:
Doctrine and Covenants 131:5
5 (May 17th, 1843.) The more sure word of prophecy means a man’s knowing that he is sealed up unto eternal life, by revelation and the spirit of prophecy, through the power of the Holy Priesthood.
We of the Church of Jesus Christ of Latter-day Saints believe that the sealing of the 144,000 mentioned in Revelation 7 is a sealing up unto eternal life. Once they are sealed, they go forth to bring others unto the Church of the Firstborn that they may receive the same blessing. Those of the Church of the Firstborn are those who are sealed up unto eternal life by having the more sure word of prophecy of having their calling and election made sure. When a person reaches the level of righteousness to be sealed unto eternal life, he/she can receive the blessing of having personal communion with the Lord Jesus Christ and the Father as a second comforter in their lives.
John 14:18, 21, 23
18 I will not leave you comfortless: I will come to you.
21 He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him.
23 Jesus answered and said unto him, If a man love me, he will keep my words: and my Father will love him, and we will come unto him, and make our abode with him.
We believe that those who love God and keep his commandments can grow in spiritual truth and strength to the point where God has tested them and found that they will do whatever God would have them do even if it means laying down their life for him. When one has reached a similar point in his/her life and has grown in truth and knowledge that he/she can receive the Second Comforter in his/her life where he/she can have direct revelation from the Father and Son in his/her life and have Jesus appear unto them. This is a blessing of those who have their calling and election made sure and have the more sure word of prophecy. We believe this is what Peter was speaking of in the first chapter of his second epistle.