In short, the Master offers not only to change us, but to exchange with us—to exchange his goodness for our sin (see 2 Corinthians 5:21; Galatians 3:13; Philippians 3:8-9; Hebrews 2:9). He came not only to cleanse our souls but also to fill our souls. He came not only to wipe away the stains but also to endow us with his righteousness and his power. He came to perfect us. Thus it is that those who go to the celestial kingdom are those who are “just men [and women] made perfect through Jesus the mediator of the new covenant, who wrought out this perfect atonement through the shedding of his own blood” (D&C 76:69, italics added).
“Perfect in Christ”
Spiritually, this is what happens when we enter into the covenant relationship with our Savior. We have liabilities, he has assets. He proposes to us a covenant relationship. I use the word ’propose’ on purpose because it is a marriage of a spiritual sort that is being proposed. That is why he is called the Bridegroom. This covenant relationship is so intimate that it can be described as a marriage. I become one with Christ, and as partners we work together for my salvation and my exaltation. My liabilities and his assets flow into each other. I do all that I can do, and he does what I cannot yet do. The two of us together are perfect. (“Believing Christ: A Practical Approach to the Atonement,” 1989-90 BYU Devotional and Fireside Speeches, pp. 120-21.)