Interestingly, Peter does not define what he means with the word he uses here for love (agape). That can only mean that Peter is confident that his readers have earlier been told what love is and what it looks like. Inasmuch as “love” gets to the very heart of the gospel (John 3:16; John 13:1–2; John 15:12–13; Philippians 2:1–11; 1 Corinthians 13:1–13), we may take for granted that Peter’s readers had had this love from God clearly explained to them when they first heard the gospel. In fact, that also explains why they “love him” (1 Peter 1:8).
22 Having purified your souls by your obedience to the truth for a sincere brotherly love, love one another earnestly from a pure heart,