I think many of us fall victim to the fallacious idea that when we arrive at judgment day, we will stand before God as imperfect beings, full of sins that need to be cleansed, and then magically transformed into a perfected god-like individual. From what I understand, in order to retain the presence of the Father (a wondrous and terrible gift that all receive, despite levels of righteousness [Hel. 14:16-18]), we must come as already perfected beings who have overcome all things through the Atonement of Christ.
Heavenly Father's laws are immutable, "But behold, I say unto you, the kingdom of God is not filthy, and there cannot any unclean thing enter into the kingdom of God" (Nephi's use of chiasmus is ubiquitous and shows emphasis on particular principles of import) (1 Ne. 15:34). In order to enjoy eternal life, the kind of life that God enjoys, we must be obedient to that law of heaven. In other words, God's plan demands perfection and nothing short of it.
Of course, we are aware that "all have sinned, and come short of the glory of God" (Romans 3:23), and Heavenly Father is aware of that too. He is aware that our sinning damns us, so in His infinite knowledge He provided a Savior, whose Atonement has the unfathomable power to cleanse and perfect. We are given a probationary period (one on probation does not evade the consequences of his actions but merely has them postponed, unless he is compliant with the conditions of the probation). The Father, in great mercy, has given us time to become perfect.
The Savior commanded us to "Be ye therefore perfect, even as your Father which is in heaven is perfect" (Matt. 5:48). The Greek word for perfect, telios, means complete, finished, or fully developed. I find it interesting that He doesn't include Himself in this commandment here, but does when He gives it to the Nephites (3 Ne. 12:48). Even the Savior, Himself, was not magically transformed overnight into a perfected, fully developed God. Rather, John says "He received not of the fulness at first, but continued from grace to grace, until he received a fulness" (D&C 93:13). So must our journey be.
Mount of Beatitudes, the setting for Christ's "Sermon on the Mount"
Some give a monetary metaphor for Christ's Atonement (e.g. "You give all you have and the Savior will make up the difference."). While I think this is great for a basic understanding, I think it ultimately falls short. Jesus Christ does not merely make up the difference. Rather, He is the difference. He doesn't just fill in what we can't do, but actually empowers us to have the capabilities to do it ourselves. Elder Bednar describes these two powerful aspects of the Atonement: the redeeming power, and the enabling power: "I suspect that many Church members are much more familiar with the nature of the redeeming and cleansing power of the Atonement than they are with the strengthening and enabling power... The gospel of the Savior is not simply about avoiding bad in our lives; it also is essentially about doing and becoming good. And the Atonement provides help for us to overcome and avoid bad and to do and become good. Help from the Savior is available for the entire journey of mortality - from bad to good to better and to change our very nature."
I think this is so critical. God does not want us to be imperfect creatures who have simply been washed clean. He wants our natures to change, for us to become, truly become, what we were designed to become: "priests and kings, [priestesses and queens] who have received of his fulness, and of his glory... Wherefore, as it is written, they are gods, even the sons [and daughters] of God" (D&C 76:56,58).
This may seem exhausting and even impossible. I have a hard time studying the scriptures consistently or waking up early and this is what is expected of me?! However, I know, like Nephi that "the Lord giveth no commandments unto the children of men [including perfection], save he shall prepare a way for them that they may accomplish the thing which he commandeth them" (1 Ne. 3:7). Did we get that? "That they may accomplish the thing which he commandeth them." We can achieve perfection and are ultimately responsible to do so. We are utterly unable to do this on our own, but Christ has prepared a way! We can access the infinite redeeming and enabling powers of the Atonement through faithfulness to the covenants that bind us to Him. We can do it! The Savior wouldn't have given us the commandment if He didn't think we could.
The "City of David," the Jerusalem of Nephi and Lehi's time.
"Man's chief concern in life should not be the acquiring of gold or fame, or of material possessions. It should not be the development of physical prowess, nor of intellectual strength, but his aim, the highest in life, should be the development of a Christlike character." -David O. Mckay
"The greatest and most important labor we have to perform is to cultivate ourselves." - Brigham Young
"Your greatest work, your most important creation, will be you." - Robert Jensen
God has given us time. We are not expected to become perfect in this mortal existence, although the scriptures do give examples of individuals who achieved some sort of perfection while here: "Noah was a just man, and perfect in his generation; and he walked with God, as did also his three sons, Shem, Ham, and Japheth" (Moses 8:27, Gen. 6:9); "There was a man in the land of Uz, whose name was Job; and that man was perfect and upright, and one that feared God, and eschewed evil" (Job 1:1); "He (Seth) was a perfect man" (D&C 107:43).
Perfection is a process, but one that we can and must work on. "Ye are not able to abide the presence of God now, neither the ministering of angels; wherefore, continue in patience until ye are perfected" (D&C 67:13). As we continue living our lives faithfully, we will come to realize that perfection is possible through Jesus Christ. The Atonement is powerful enough to perfect us.
The ultimate realization of our perfection will be in our glorious resurrection. Speaking of those who overcame all things by faith, the Lord said, "These are they who are just men made perfect through Jesus the mediator of the new covenant, who wrought out this perfect atonement through the shedding of his own blood. These are they whose bodies are celestial, whose glory is that of the sun, even the glory of God, the highest of all" (D&C 76:69-70). It is after this resurrection, after perfection, that we can be brought before our Heavenly Father, and enjoy His presence for eternity. We can completely fulfill His demands for justice because of the mercy of His Son. We can and must achieve perfection.