Sometimes the thought occurs to me, 'what if I'm a fake?'
John 6:37 spells out a clear teaching of Christianity: those who belong to God will never fall away.
1 John 2:19 buttresses this point: those who fall away from the faith were never truly saved to begin with.
Matthew 7:21-23 says plainly that many who go to hell will actually be convinced that they are Christians, because they call Jesus "Lord."
For those who like powerful video/audio presentations, the one below might capture your interest. Be warned, it might be scary to some. If the above verses are enough to give you the idea that there are fake Christians, you can skip the video if you're pressed for time.
Here's the deal: if you're saved, you cannot become unsaved (Romans 8:38-39). And if you're saved, you'll pursue a godly life and subsequently sin less and less in your daily life. You never become perfect, so you will sin -- but you fall into it, you don't dive.
The question that goes through my mind is, how can you measure the distinction, practically? After sinning, I naturally feel bad. But feeling bad isn't necessarily repentance. Repentance would involve a complete about-face, not merely the admission of guilt, followed by continued indulgence.
When a long time has gone since the last time you sinned in a particular way, you feel good about it. Living righteously, I suppose, improves your confidence. David in Psalm 51:12 seems to indicate that what we lose when we sin is not our salvation, but our confidence in knowing that we have it. We lose our assurance. Not the seal itself, but our personal assurance that we have been sealed.
The simple solution seems to be to make a habit of repenting as soon as possible after you sin, praying for strength to resist it in the future, and getting back in the mindset of trying to avoid it. You'll never succeed (1 John 1:8-10), but the mark of a Christian is a continual mortification of the flesh--it's not something you do once and never again. It's a persistent attitude of the heart that leads to increasing grief over sin and an increasing gratitude toward God for His forgiveness in spite of our continual sin.
I'm encouraged when I recently stood back and noticed that what makes me the most upset about when I commit a sin -- what grieves me the most -- is not its severity, nor even its perpetuity, but the fact that the very act of it, no matter how great or small the sin, is a proclamation of ingratitude toward God for His forgiveness. It is like when you are in a close relationship with someone -- a close family member or romantic partner -- and you do something to hurt them. You're not upset primarily because you realize that what you did was ontologically wrong -- you're upset because what you did hurt the one you love. Now, God cannot be hurt in the sense that He is weakened. But that doesn't mean He can't be disrespected. And the One least deserving in all of existence to be disrespected must surely be God.
Consequently, the thing I most look forward to upon death is the final freedom from sin. To no longer have it be part of my nature to reject God and the blessing that is God. Heaven will be great, I'm sure. And exploring the New Earth will be an enjoyment. But what will make it fantastic is that my relationship with my heavenly Father will no longer have the potential to be harmed by my selfishness.
Until that day, there is work to done here on earth.
~ Rak Chazak