You will need to decide whether to treat v. 21 as a part of this paragraph or whether to treat it as a separate sermon. I opted for the latter in my preaching through 1 John, based on linguistic reasons. If you go with the former approach, v. 21 functions as the exhortation and final appeal with 5:13-20 functioning as the grounds or reason for the closing appeal.
Verse 13 functions as the dominant claim or point: knowing that you have eternal life. The rest of the paragraph is sub-divided into the following units:
The sermon would have one main point (v. 13), followed by 4 sub-points, each further describing what is true of those who possess eternal life.
I. Christians know that they have eternal life – (13)
A. Because of answered prayer – (14-17)
B. Because we have God’s protection – (18)
C. Because we belong to God – (19)
D. Because we know God, are united to him and Jesus, and have eternal life – (20)