One thing a Christian can have no doubt about is that he is saved. God said that’s how it is, so who is going to tell God that He is wrong?
It can’t be stated more clearly than in First John 5:13,
These things have I written unto you that believe on the name of the Son of God; that you may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Of course, if you are relying on anything else other than God’s immutable promise, like trying to live a good life to outweigh the bad in your life, you can’t know for sure if you’ll go to heaven. And though you might doubt your future, the Bible says that you will never get to heaven by trying to be good enough.
Ephesians 2:8-9 says,
For by grace are ye saved through faith; and that not of yourselves: [it is] the gift of God: Not of works, lest any man should boast.
God says trying to get to heaven by living a life that is good enough is useless. Only a vain person would think that he is good enough to get to heaven. He won’t make it.
The Apostle Paul writes in Romans 3:20,
Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law [is] the knowledge of sin.
In Romans 3:28, Paul writes,
Therefore we conclude that a man is justified by faith without the deeds of the law.
To be saved by works would mean that God owes you a reward for doing good enough. And to be good enough means that you could boast about it. But Romans 3:27 says,
Where [is] boasting then? It is excluded. By what law? of works? Nay: but by the law of faith.
On the other hand, if you are confident in the work and promise of Jesus Christ, then you can be confident in your salvation because of what He did. If you believe in His promise you can know that you are saved.
First John 5:13 says,
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Do you believe that you must rely on Christ 100%? You do if you believe Him.