Ready for a scary fact? If you think you're a Christian, you might be wrong. If you agree with me that this is a scary fact, however, you might be right after all. Why? Because a non-Christian wouldn't care.
Test yourself, as Paul said. Are you really a Christian? Let's ask a few questions.
Why do you believe you're a Christian? A good answer would be because you recognise that you were an enemy of God because of your sin, but God, in all His glory, loved you so much that He sent Jesus to take His wrath in your stead so that you can be redeemed by His righteousness and walk in His ways. Your active faith in that truth is what makes you a Christian.
But what if your answer is different than that? What if you believe you're a Christian because you do good things? Matthew 7:21-23 doesn't speak very highly of those who rely on their own works. If you ask Jesus to judge you by your works, rather than your faith, He will. But that will include your bad works, the wages of which is death (Romans 6:23) and so Jesus will say "depart from me, lawless one".
What if you go to church every Sunday? Romans 2:28 says one is not a Jew who is one outwardly, but who is one inwardly. The same applies to Christianity (which is really just a more complete Jewish faith), so sitting in a church doesn't make you a Christian any more than sitting in a garage makes you a mechanic.
But that's cool for you Christians who think "awesome, I sin like it's nothing, but I have faith in Jesus, so I'm good." I fear for you most of all. I could bring a plethora of scriptures here. What about James 2:18-19? Even demons believe in Jesus, but they are not saved. If your faith is real, prove it, because faith is more than just believing God is real, it's about believing He is worthy. Matthew 5:6 says blessed are those who hunger and thirst for righteousness, for they shall be filled. If I'm hungry, I go looking for food and eat it. If I'm thirsty, I seek a drink. If you truly hunger and thirst for righteousness, by what logic do you reach the conclusion that sin is a valid diet?
But what if you have no sin? Well then, you, my friend, are either Jesus, or do not exist. 1 John 1:8: if you say you have no sin, you are deceiving yourself, and the truth is not in you. If you think you're sinless, you're not Christian, you're just plain arrogant.
Do you see yourself at any point in this article? As the saying goes, if the shoe fits, wear it. But while you're wearing those shoes, take a trip to Calvary. When you're there, take those shoes right off and put them at the foot of the cross, because if you're "disqualified" for the indwelling now, that doesn't mean you will be in 5 minutes. After testing yourself, if you fail the test, God is perfect in grace, because He loves you. Therefore, you have the ability to try again. And again. Until you die, and face Him in judgement, there is time to repent. But there may not be tomorrow. Don't wait.