The Apostle Paul said in Galatians 1:8-9, "But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed."
What is the Gospel? That Jesus died for our sins and rose again on the third day. Anyone that believes that and confesses with their mouth, they will be saved (Romans 10:9-10, 1 Cor. 15:1-4).
The question then becomes "can we lose that salvation?" The Bible will always interpret the Bible, so let's allow God's incorruptible Word speak for itself.
When Jesus was speaking to Nicodemus in John chapter 3, He told Nicodemus that a person must be born again in order to enter the Kingdom of God. The pharisee, confused, asked Jesus how a person can re-enter and exit the womb a second time, not understanding the logistics of what He was talking about. Jesus replied that a person must be born of water (the physical birth) and of the Spirit (the spiritual birth). Everything in this world has a spiritual and natural parallel (Heb. 9:23-24). Jesus Himself was born of the Holy Spirit and through the physical body of Mary (Matt. 1:18-25). Jesus Himself is both God and man (Col. 2:9). Just as a person cannot reenter the physical womb, their spirit cannot reenter the spiritual womb.
Salvation is more than just the forgiveness of sins. When a person accepts Jesus as their Lord and Savior a transformation takes place. The Bible says that we become a new creation. 2 Corinthians 5:17 says "therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new". We are a new creation and the old creation has DIED! If a person can lose their salvation, can they regain it? If so, that would mean that we all have the same power that God does to resurrect the dead. Paul clearly states that the old creature has "passed away".
What else happens when a person becomes born again? The Holy Spirit enters us. Jesus said in John 14:16 that the Holy Spirit will "abide with you forever". How long is forever? It is forever! Additionally, Ephesians 4:30 states that we are "sealed unto the day of redemption". The day of redemption is the Rapture. If you are unfamiliar with the Rapture of the Church please click here.
There are many verses that state that our salvation is forever. The specific term used is "eternal". How long is eternity? The same length of time that forever is! Look at John 3:16, John 10:28, and 1 John 5:13 for instance.
Before I discuss verses that cause confusion, I want to make it clear that if a person teaches that believers can lose their salvation, then they are calling God a liar. When Jesus was on the cross, He proclaimed "it is finished" (John 19:30). What was finished? Taking on the sins of the entire world past, present, and future (Rom. 3:23-25, 2 Cor. 5:18-19). How can a person lose their salvation if all of their sins were nailed to the cross? They cannot!
There are some verses that appear to muddy the waters a little bit. Let's start with Hebrews 6:4-6 in which the Apostle Paul wrote:
"For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
Let me offer a few phrases that will make this passage perfectly clear. "I would never drive my car again IF it were possible for me to run like The Flash." "It would be odd for me to book a flight through Delta Airlines IF it were possible for me to fly like Superman." Paul was clearly saying that IF it were possible for a Christian to lose his/her salvation, then there is no possible way for them to accept Christ ever again. People that proclaim that a Christian can lose their salvation cannot also proclaim that a person can get it back. That isn't Biblical.
2 Peter 2:20 is also often used to justify that a person can lose their salvation. It is taken out of context though. Look at what proverb he quotes a few verses later: Proverbs 26:11. Peter is talking about how foolish it is for a Christian to try and go back to his or her old life. If you give up a sin which causes you to lose friends, and end up going back to that sin, is your life better? Of course not. Not only are you depressed and guilt-ridden, but those friends that you lost will probably call you a hypocrite which would make you feel even worse. When a dog throws up, the dog is getting rid of what is upsetting its stomach (symbolic of casting off a sinful life once we accept Christ). When the dog eats its vomit, it usually gets sick again and throws back up. That was what Peter was saying which is why he quoted that specific proverb.
No believer can lose their salvation. The Bible makes it perfectly clear and when a person rightly divides the Word of Truth (2 Tim. 2:15), they cannot possibly come to the conclusion that a believer can lose their salvation.
If you would like to dive into the one time period in which it is possible to permanently reject Christ, please read about the seven dispensations found in the Bible here and scroll down to the mark of the beast.