For it is by grace you have been saved, through faith - and this not from yourselves, it is the gift of God - not by works, so that no one can boast. (Ephesians 2:8-9)
Salvation. Eternal life. Heaven. It's something that most people either think about, don't care about, are confident about, are confused about, or are concerned about. I've had many conversations with people over the years about the subject, and they generally fall into one of the above categories.
Most of my life, I've felt confident about my eternal future. I base that confidence on the very words of Christ and live by the verse, John 3:16: "For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life." I also fully acknowlege my sinfulness and realize that my salvation is not based on my own individual merit, but rather the perfect sacrifice of my Lord and Savior Jesus Christ.
The main reason my confidence is as strong as it is, is that through Christ, I believe I can be reconciled to a perfect God that detests sin. Knowing full well that I am a sinner (each week the list of sins only grows), I take joy in knowing that each of those sins; past, present, and future have been fully paid for by Christ.
Many people that have a lesser confidence in their eternal future usually either question the authority of the bible, the authority of Christ, struggle with what will happen to good people who do not believe in Christ, or refuse to believe that God could ever forgive them for what they've done in the past. To those who question the authority of bible, I challenge them to read it with an open mind. Most people who question the authenticity of the bible, have never even picked it up. To those who question the authority of Christ, I ask you to read about him and come to know him. No relationship can be built between two people (physical or spiritual) without knowledge and communication. To those who question what will happen to good people who do not believe in Christ, I have no easy answer. But the bible does state that "to whom much is given, much is required." I believe that people who had a conscious knowledge of Christ, yet openly rejected him, will be held more accountable than those who did not. Lastly, to those who believe that God could never forgive them, draw strength from the thief who repented in his last dying breath. He said to Jesus, "remember me when you come into your kingdom." Jesus answered, "I tell you the truth, today you will be with me in paradise."