To maintain plainness, we opt for the word “sinlessness” over the phrase “perfect in Christ,” as the definition of the latter can too easily be hijacked to mean “a magical transformation from a sinner to a saint,” while the former cannot.
Sinlessness is to live in such a way that every action and desire you have, in every moment, aligns completely with your sincere present understanding of the character of God—what you imagine he would do in your place.
Sinlessness is required in order to receive forgiveness of past sins. The scriptures call this justification.
Sinlessness is also required to have eternal life. The scriptures call this sanctification.
John the Revelator described true disciples of God as those who purify themselves, becoming like Jesus by not sinning and by keeping Jesus’ commandments:
1 Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.
2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.
3 And every man that hath this hope in him purifieth himself, even as he is pure.
4 Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.
5 And ye know that he was manifested to take away our sins; and in him is no sin.
6 Whosoever abideth in him sinneth not: whosoever sinneth hath not seen him, neither known him.
7 Little children, let no man deceive you: he that doeth righteousness is righteous, even as he is righteous.
8 _He that committeth sin is of the devil_; for the devil sinneth from the beginning. For this purpose the Son of God was manifested, that he might destroy the works of the devil.
9 _Whosoever is born of God doth not commit sin_; for his seed remaineth in him: and he cannot sin, because he is born of God.
10 In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother. (1 John 3)
Anyone living anything short of sinlessness is destined for hell—to pay the price of their past sins and to eternally contend with the knowledge of what they could have had if they had not turned away from the opportunities for improvement they had in life.