"...if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from every sin." (1:7). "...if anyone obeys his Word, God's love is truly made complete in him... Whoever claims to live in him must walk as Jesus did." (2:5-6).
The antichrist denies that Jesus is the Christ (the Messiah; the Deliverer) - 2:22; see 2:18-27 & 4:1-6.
John really puts it on the line here! See 2:28-3:24 and 4:7-5:21. "If you know that he is righteous, you know that everyone who does what is right has been born of him." (2:29). This is the message you heard from the beginning, "And this is his command: to believe in the name of his Son, Jesus Christ, and to love one another as he commanded us." (3:23). "We love because he first loved us." (4:19). "We know that anyone born of God does not continue to sin..." (5:16).