1 John 2:18-19 says,
Dear children, this is the last hour; and as you have heard that the antichrist is coming, even now many antichrists have come. This is how we know it is the last hour. They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us."
If you aligned yourself with Christianity, but then fell away for some reason, John states here that you "did not really belong". These people are against Christ still - what John terms "antichrists" (not AntiChrist, but simply a person against Christ). Christians belong with other Christians, as they are the only ones having been adopted into God's family. Jesus states it this way, "No one, having put his hand to the plow, and looking back, is fit for the kingdom of God" (Luke 9:62). If you're longing for your old life, you don't belong
1 John 2:22-23 says,
Who is the liar? It is the man who denies that Jesus is the Christ. Such a man is the antichrist-he denies the Father and the Son. No one who denies the Son has the Father; whoever acknowledges the Son has the Father also.
And what does it mean to deny that Jesus is the Christ? The Christ is the person whom the Old Testament kept talking about, prophesied to be the Saviour of the world through taking God's wrath and punishment that we deserve on the Cross, such as in Isaiah 53. The Jews, for instance, deny that Jesus was the Christ and are still waiting for the Messiah. They then have no Saviour whatsoever.
(Be honest with yourself! Data is stored temporarily for your eyes only. This also means you can go back and change your answers.)