Verse 19 warns that no unclean thing may enter the kingdom of God. Verse 20 explains that we become “spotless” by repentance, baptism, and the gift of the Holy Ghost. Those are the rites that witness our complete understanding and acceptance of the atonement.
Reference: 2 Chronicles 23:19, though probably written after Lehi left Jerusalem, explains a similar rule: “And he [David] set the porters at the gates of the house of the Lord, that none which was unclean in any thing should enter in.” The “house of the Lord” is the temple, not heaven, but it can (and symbolically always does) welcome his presence. Thus, to enter the temple is symbolically to enter the Lord’s presence. If cleanliness here is so important, how much more for his actual presence?