Many people in the church lack assurance that they are a Christian. There are many reasons for that. It could be bad teaching like hearing a pastor say, "If you remember the day and hour of you prayed a prayer" or "You remember how old you were when you came to Christ." It could unconfessed sin that causes you to doubt.
Whatever the reason, many Christians doubt their salvation. How can one be assured of their salvation? Kevin DeYoung has written three signs that you can determine for yourself if you a Christian:
The first sign is theological. You should have confidence if you believe in Jesus Christ the Son of God (5:11-13). John doesn’t want people to be doubting. God wants you to have assurance, to know that you have eternal life. And this is the first sign, that you believe in Jesus. You believe he is the Christ or the Messiah (2:22). You believe he is the Son of God (5:10). And you believe that Jesus Christ has come in the flesh (4:2). So if you get your theology wrong about Jesus you will not have eternal life. But one of the signs that should give you confidence before God is that you believe in his only Son Jesus Christ our Lord (4:14-16; 5:1, 5).
The second sign is moral. You should have confidence if you live a righteous life (3:6-9). Those who practice wickedness, who plunge headlong into sin, who not only stumble, but habitually walk in wickedness-should not be confident. This is no different than what Paul tells us in Romans 6 that we are no longer slaves to sin but slaves to righteousness and in Galatians 5 that those who walk in the flesh will not inherit the kingdom. This is no different than what Jesus tells us in John 15 that a good tree cannot bear bad fruit and a bad tree cannot bear good fruit. So if you live a morally righteous life you should have confidence (3:24). And lest this standard make you despair, keep in mind that part of living a righteous life is refusing to claim that you live without sin and coming to Christ for cleansing when you do sin (1:9-10).
The third sign is social. You should have confidence if you love other Christians (3:14). If you hate like Cain you do not have life. But if your heart and your wallet are open to your brothers and sisters eternal life abides in you. One necessary sign of true spiritual life is that we love one another (4:7-12, 21).
These are John’s three signposts to assure us that we are on the road that leads to eternal life. These are not three things we do to earn salvation, but three indicators that God has indeed saved us. We believe in Jesus Christ the Son of God. We live a righteous life. We are generous toward other Christians. Or we can put it this way: we know we have eternal life if we love Jesus, we love his commands, and we love his people. No one of the three is optional. All must be present in the Christian, and all three are meant to be signs for our assurance (see 2:4, 6; 4:20; 5:2).
John belabors the same points again and again. Do you love God? Do you love his commands? Do you love his people? If you don’t, it’s a sign you have death. If you do, it’s sign that you have life. And that means confidence instead of condemnation.
Am I Really A Christian? by Mike McKinley
How Can I Be Sure?: And Other Questions about Doubt, Assurance and the Bible by John Stevens