So, what's 1 John's answer to the question "How do I know if I'm a Christian"?
The answer is twofold.
First, there is a confessional aspect. A Christian is one who confesses Jesus as the Son of God. For example:
The second part of the answer "Who is a Christian?" turns to love. We know we are Christians if we love. For example:"Who is the liar? It is whoever denies that Jesus is the Christ. Such a person is the antichrist—denying the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also." (2.22-23)
"Dear friends, if our hearts do not condemn us, we have confidence before God and receive from him anything we ask, because we keep his commands and do what pleases him. And this is his command: to believe in the name of his Son, Jesus Christ..." (3.21-23a)
"Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. This is how you can recognize the Spirit of God: Every spirit that acknowledges that Jesus Christ has come in the flesh is from God, but every spirit that does not acknowledge Jesus is not from God." (4.1-3a)
"And we have seen and testify that the Father has sent his Son to be the Savior of the world. If anyone acknowledges that Jesus is the Son of God, God lives in them and they in God." (4.14-15)
"Everyone who believes that Jesus is the Christ is born of God, and everyone who loves the father loves his child as well." (5.1)
"Who is it that overcomes the world? Only the one who believes that Jesus is the Son of God." (5.5)
"Whoever believes in the Son of God accepts this testimony. Whoever does not believe God has made him out to be a liar, because they have not believed the testimony God has given about his Son. And this is the testimony: God has given us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life." (5.10-12)
"I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life." (5.13)
"Anyone who claims to be in the light but hates a brother or sister is still in the darkness. Anyone who loves their brother and sister lives in the light, and there is nothing in them to make them stumble." (2.9-10)
"We know that we have passed from death to life, because we love each other. Anyone who does not love remains in death. Anyone who hates a brother or sister is a murderer, and you know that no murderer has eternal life residing in him." (3.14-15)
"This is how we know what love is: Jesus Christ laid down his life for us. And we ought to lay down our lives for our brothers and sisters. If anyone has material possessions and sees a brother or sister in need but has no pity on them, how can the love of God be in that person? Dear children, let us not love with words or speech but with actions and in truth." (3.16-18)
"Dear friends, let us love one another, for love comes from God. Everyone who loves has been born of God and knows God. Whoever does not love does not know God, because God is love." (4.7-8)
"God is love. Whoever lives in love lives in God, and God in them. This is how love is made complete among us so that we will have confidence on the day of judgment: In this world we are like Jesus." (4.16b-17)
"Whoever claims to love God yet hates a brother or sister is a liar. For whoever does not love their brother and sister, whom they have seen, cannot love God, whom they have not seen. And he has given us this command: Anyone who loves God must also love their brother and sister." (4.20-21)
I'm struck by the strongly behavioral aspect regarding assurance on Judgement Day. Confession, while critical, isn't sufficient. Love is necessary and required. 1 John 4.16-17 sums it up nicely. How can we be confident on the Day of Judgment? We can have confidence if we walk in the world as Jesus walked. Love is what "casts out fear" regarding judgment. Because God is love, and whoever lives in love lives in God.
So these are the two assurances in 1 John. Who is a Christian? The one who confesses Jesus Christ as the Son of God and the one who loves as Jesus loved.