This is a very good question for it is not something that just jumps out at us upon reading scripture, as such, it is a point of great misunderstanding. Hopefully I'll explain this doctrine in an understandable fashion:
The doctrine is rooted in the following verses, of which I'll add some commentary as we go; the verses are:
Heb. 4:14; 8:1, 2; Lev. 16:2, 29; Heb. 9:23, 24; Dan. 8:14; 9:24-27; Rev. 14:6, 7; 22:11.
Heb. 4:14 - Seeing then that
we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.
Heb. 8:1, 2 - Now of the things which we have spoken this is the sum: We have such an high priest, who is set on the right hand of the throne of the Majesty in the heavens;
2 A minister of the sanctuary,
and of the true tabernacle, which the Lord pitched, and not man.
Lev 16:2-29 - And the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the vail before the mercy seat, which is upon the ark; that he die not: for I will appear in the cloud upon the mercy seat. 3 Thus shall Aaron come into the holy place: with a young bullock for a sin offering, and a ram for a burnt offering.
4 He shall put on the holy linen coat, and he shall have the linen breeches upon his flesh, and shall be girded with a linen girdle, and with the linen mitre shall he be attired: these are holy garments; therefore shall he wash his flesh in water, and so put them on.
5 And he shall take of the congregation of the children of Israel two kids of the goats for a sin offering, and one ram for a burnt offering.
6 And Aaron shall offer his bullock of the sin offering, which is for himself, and make an atonement for himself, and for his house.
7 And he shall take the two goats, and present them before the LORD at the door of the tabernacle of the congregation.
8 And Aaron shall cast lots upon the two goats; one lot for the LORD, and the other lot for the scapegoat.
9 And Aaron shall bring the goat upon which the LORD's lot fell, and offer him for a sin offering.
10 But the goat, on which the lot fell to be the scapegoat, shall be presented alive before the LORD, to make an atonement with him, and to let him go for a scapegoat into the wilderness.
11 And Aaron shall bring the bullock of the sin offering, which is for himself, and shall make an atonement for himself, and for his house, and shall kill the bullock of the sin offering which is for himself:
12 And he shall take a censer full of burning coals of fire from off the altar before the LORD, and his hands full of sweet incense beaten small, and bring it within the vail:
13 And he shall put the incense upon the fire before the LORD, that the cloud of the incense may cover the mercy seat that is upon the testimony, that he die not:
14 And he shall take of the blood of the bullock, and sprinkle it with his finger upon the mercy seat eastward; and before the mercy seat shall he sprinkle of the blood with his finger seven times.
15 Then shall he kill the goat of the sin offering, that is for the people, and bring his blood within the vail, and do with that blood as he did with the blood of the bullock, and sprinkle it upon the mercy seat, and before the mercy seat:
16 And he shall make an atonement for the holy place, because of the uncleanness of the children of Israel, and because of their transgressions in all their sins: and so shall he do for the tabernacle of the congregation, that remaineth among them in the midst of their uncleanness.
17 And there shall be no man in the tabernacle of the congregation when he goeth in to make an atonement in the holy place, until he come out, and have made an atonement for himself, and for his household, and for all the congregation of Israel.
18 And he shall go out unto the altar that is before the LORD, and make an atonement for it; and shall take of the blood of the bullock, and of the blood of the goat, and put it upon the horns of the altar round about.
19 And he shall sprinkle of the blood upon it with his finger seven times, and cleanse it, and hallow it from the uncleanness of the children of Israel.
20 And when he hath made an end of reconciling the holy place, and the tabernacle of the congregation, and the altar, he shall bring the live goat:
21 And Aaron shall lay both his hands upon the head of the live goat, and confess over him all the iniquities of the children of Israel, and all their transgressions in all their sins, putting them upon the head of the goat, and shall send him away by the hand of a fit man into the wilderness:
22 And the goat shall bear upon him all their iniquities unto a land not inhabited: and he shall let go the goat in the wilderness.
23 And Aaron shall come into the tabernacle of the congregation, and shall put off the linen garments, which he put on when he went into the holy place, and shall leave them there:
24 And he shall wash his flesh with water in the holy place, and put on his garments, and come forth, and offer his burnt offering, and the burnt offering of the people, and make an atonement for himself, and for the people.
25 And the fat of the sin offering shall he burn upon the altar.
26 And he that let go the goat for the scapegoat shall wash his clothes, and bathe his flesh in water, and afterward come into the camp.
27 And the bullock for the sin offering, and the goat for the sin offering, whose blood was brought in to make atonement in the holy place, shall one carry forth without the camp; and they shall burn in the fire their skins, and their flesh, and their dung.
28 And he that burneth them shall wash his clothes, and bathe his flesh in water, and afterward he shall come into the camp.
29 And this shall be a statute for ever unto you: that in the seventh month, on the tenth day of the month, ye shall afflict your souls, and do no work at all, whether it be one of your own country, or a stranger that sojourneth among you:
Heb. 9:23 - It was therefore necessary that the
patterns of things in the heavens should be purified with these; but the heavenly things themselves with better sacrifices than these.
Dan. 8:14 - And he said unto me, Unto
two thousand and three hundred days;
then shall the sanctuary be cleansed.
Dan. 9:24-27 - Seventy weeks are determined upon thy people and upon thy holy city, to finish the transgression, and to make an end of sins, and to make reconciliation for iniquity, and to bring in everlasting righteousness, and to seal up the vision and prophecy, and to anoint the most Holy.
25 Know therefore and understand, that from the going forth of the commandment to restore and to build Jerusalem unto the Messiah the Prince shall be seven weeks, and threescore and two weeks: the street shall be built again, and the wall, even in troublous times.
26 And after threescore and two weeks shall Messiah be cut off, but not for himself: and the people of the prince that shall come shall destroy the city and the sanctuary; and the end thereof shall be with a flood, and unto the end of the war desolations are determined.
27 And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate.
Rev. 14:6, 7 - And I saw another angel fly in the midst of heaven, having the everlasting gospel to preach unto them that dwell on the earth, and to every nation, and kindred, and tongue, and people, 7 Saying with a loud voice, Fear God, and give glory to him;
for the hour of his judgment is come: and worship him that made heaven, and earth, and the sea, and the fountains of waters.
Rev. 22:11 - He that is unjust, let him be unjust still: and he which is filthy, let him be filthy still: and he that is righteous, let him be righteous still: and he that is holy, let him be holy still.
Ok, now that we have the applicable verses I'll see if I can answer your questions:
Basically, we believe that there will be a point in time where Christ, our heavenly High Priest (
Heb. 4:14 & Heb. 8:1, 2), will have to close the heavenly tabernacle in heaven so that judgment can be passed. Judgment cannot be made unless there is an examination done on every person who has ever lived, this is how names are come to be blotted out in the book of Life. As you know, the earthly tabernacle was just a shadow of the true tabernacle in heaven; and it functions in a similar manner - the big difference is that Christ is the atoning blood, not animals as it was in the Old Covenant. Christ is the High Priest, the minister, of the heavenly tabernacle and just like the example God gave us with the Levitical priests, there is a time when Christ will have to cleanse the heavenly tabernacle; we believe this occurred in 1844 as the fulfillment of
Dan 8:14, as the 2,300 prophetic years talks about. What that means to us today is that essencially we are on borrowed time - just as in the days of Noah. God gave about 130 years (if memory serves) for Noah to try an convince the people of His coming wrath - but they wouldn't heed. We feel we are in a similar time for God's wrath and judgment is fast approaching.
The rather lengthy post of
Lev 16:2-29 is there to explain the functions of the earthly tabernacle - which were the pattern for what actually occurs in heaven. It is important to know the minute details of this atonement system for, just as Heb. 9:23 indicates, it is patterned after the heavenly tabernacle and the "better sacrifice" is our lamb, Jesus Christ. If we don't fully understand the earthly tabernacle's function (specifically that of the scapegoat and how sin is transferred to it) then we'll never understand how atonement for sin actually occurs and when/how all sin will eventually be placed on Satan's head and sin will be no more.
As
Rev. 14:6, 7 indicates, there will be a judgment that separates the wicked from the righteous. When this occurs, just as Rev. 22:11 states, those that are wicked will remain wicked - there is no second chance, and those that are righteous will remain righteous.
For the Christian, what happens if he's not fully forgiven by the time of his death?
Christ knows the heart of each of us. If a person has strived to live a life of righteousness and always humbled himself to Christ when those times of stumbling have occurred (for we all sin) and repented of that sin and sincerely asked for forgiveness, Christ will know if that person would have repented if given the opportunity and thus their judgment will be favorable. So when you say "fully forgiven" I assume you mean that there were sins not confessed or repented of in that person's life? If so, Christ will know and understand that.
How does he receive full forgiveness?
In the exact manner described in the above answer.
Does he have assurance of this in this life?
Yes, absolutely! But that doesn't mean we can become slack in repenting and confessing of our sins. Righteousness is not a destination, it is a journey, a life devoted to Christ, accepting His free gift of grace, and following His moral law. Christ is our only hope for salvation, not works, but that doesn't give us license to willfully sin or live as we please - it is the will of the Father that must yielded to. To us, grace is not a one-time good deal; that after baptism we are forever "saved". God gave us freedom of choice; we can choose to follow Him or we can choose to follow Satan. This is a daily choice (
1 Cor 15:31) we make that God will not interfere with, else we don't really have freedom of choice. One who is righteous has the ability to turn from righteousness and fall into iniquity (this is reflected in
Eze 18:24, Isaiah 29:13, Titus 3:8, Luke 22:32, Hebrews 6:15, Hebrews 12:15, 2 Timothy 2:3, 2 Thess. 1:4, and
Matt 7:22, 23 to name a few). Therefore we must be diligent to stay on the straight path (
Matt 7:14) which leads to eternal life.
You'll notice that I haven't yet referenced
Dan. 9:24-27; I did this for a reason. You see, the Dan. 9 prophecy is key to this study, especially for those that belief there IS a second chance after the "secret rapture" - that belief is rooted in what believe is a misinterpretation of the Dan. 9 prophecy, specifically verse 27.
Would you like a quick explanation of how we interpret the
Dan. 9 prophecy?