Short answer: From a standard evangelical Christian perspective, the most likely author of 1 John (and therefore the author of 1 John 4:20–21) is the Apostle John — “the disciple whom Jesus loved” who also wrote the Gospel of John. Evangelicals hold his authorship as giving the letter apostolic authority and eyewitness reliability.

Why evangelicals identify John the Apostle as the author
– Internal similarity to the Gospel of John: 1 John shares the same vocabulary (light/darkness, life, love, truth), theology (Jesus as the incarnate Son, emphasis on fellowship with the Father), and pastoral tone as the Fourth Gospel.
– Eyewitness character: the writer speaks in ways that imply personal familiarity with Jesus and the Johannine circle (the “we” language and insistence on life in Christ). Evangelicals read this as consistent with an apostle who knew Jesus personally.
– Early church testimony: second-century writers (e.g., Irenaeus, Clement of Alexandria, Tertullian) attribute the Gospel and the Johannine epistles to John of Ephesus, and the early church consistently treated 1 John as apostolic and canonical.
– Context and setting: evangelicals place John in Ephesus late in the first century (commonly dated c. AD 85–95), writing to churches in Asia Minor troubled by false teaching (early forms of docetism/antichrist teaching) and moral confusion.

How that authorship shapes interpretation of 1 John 4:20–21
– Authority and pastoral purpose: If John the Apostle wrote it, the text carries apostolic weight. The exhortation that loving God and loving brothers are inseparable is not merely ethical advice but an apostolic test of true Christian faith.
– Connection to the Gospel: John’s Gospel emphasizes love as Jesus’ central command (e.g., John 13:34–35; 15:12). 1 John 4:20–21 restates that command as a practical test: one cannot genuinely love an unseen God while hating a seen brother.
– Polemical context: John is addressing people who may claim right belief while denying ethical love. Evangelicals read this as part of his work to expose false professors and assure believers about what real faith looks like.

Brief exegesis (what 1 John 4:20–21 says)
– Verse 20: A claim to love God is contradicted if one hates a fellow believer; such a claim is labeled “a liar.” The argument is: love of God and love of neighbor are logically and morally connected.
– Verse 21: This is presented as a commandment from God (and from Jesus): loving God requires loving one’s brother. Evangelicals emphasize that love is not optional evidence but a commanded fruit of genuine relationship with God.

Practical evangelical application
– Assurance and holiness: Love for others is a sign of saving faith and a basis for assurance (not merely an add-on).
– Church life and discipline: Churches should expect and require love among members; failure to love calls for repentance and restoration.
– Witness: Christlike love is central to evangelism — our love for each other authenticates our message about God.

Summary
Evangelicals typically identify the Apostle John (the “beloved disciple”) as the most likely author of 1 John. That authorship gives the exhortion in 4:20–21 strong apostolic authority: genuine love for God is inseparable from love for fellow Christians, and this is a commanded, assessable mark of true faith.