Short answer
– From an evangelical Christian perspective, the most likely author of 1 John (and therefore of 1 John 1:9) is the Apostle John — often called “the beloved disciple” — the son of Zebedee and one of Jesus’ inner circle.

Why evangelicals typically attribute it to John the Apostle
– Internal testimony: the opening of 1 John emphasizes eyewitness testimony (1 John 1:1–4: “we have heard, we have seen, we have looked at, and our hands have touched”), which evangelicals take as the testimony of an apostolic eyewitness.
– Early church testimony: second‑century writers (e.g., Irenaeus) identify John the Apostle as the author of the Johannine writings, a tradition Scripture interpreters in the evangelical world largely accept.
– Theological and linguistic affinity: the themes and language of 1 John (light/darkness, life, truth, love, Son of God) closely match the Gospel of John, which many evangelicals also attribute to the same apostle.
– Historical context: tradition places John ministering in Ephesus late in the first century (commonly dated c. 85–95 AD), where he would have written pastoral letters to churches in Asia Minor. Evangelicals often date 1 John in that general timeframe.

A brief word about scholarly alternatives
– Modern critical scholarship offers alternatives (e.g., a “Johannine elder” or a Johannine community), but most evangelical commentators favor apostolic authorship because of the internal claims and early patristic testimony.

What 1 John 1:9 means from an evangelical perspective
– The verse: “If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness.”
– Key evangelical emphases:
– Assurance: God’s faithfulness and justice guarantee forgiveness for those who confess; this gives believers confidence before God.
– Ongoing repentance: confession is understood as both the turning from sin (repentance) and admitting dependence on Christ’s atoning work — not a meritorious act that earns salvation, but a necessary posture of a regenerated life.
– Forgiveness and cleansing: forgiveness is real and complete; God removes guilt and purifies the believer.
– Fellowship: confession restores and maintains fellowship with God and the community (contrast the darkness of unconfessed sin in 1 John 1:5–7).
– Distinct from sacramental confession: evangelicals typically view this as direct confession to God (though private pastoral confession can be biblical and helpful), not a requirement to confess to a priest for sacramental absolution.

Pastoral application (evangelical)
– If you are a believer: practice regular, honest confession when you sin, trusting God’s promises of forgiveness and cleansing.
– If you’re investigating faith: the verse points you to Christ’s finished work — forgiveness is available through genuine confession and faith in Jesus.

If you want, I can point to specific evangelical commentaries or authors (e.g., Leon Morris, D. A. Carson, John Stott) that defend Johannine apostolic authorship and unpack 1 John 1:9 in more depth.