How? What does one have to do in order to lose it?
There are several ways:
1. Renouncing/turning your back on Christ, especially during a time of persecution. Jesus Christ says that if you deny Him before men, He will deny you before the Father on Judgment Day(Matthew 10:33).
2. Living in unrepentant, known, willful sin. Hebrews 10:26 says "For if we sin WILFULLY after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins".
The Bible means what it says and says what it means. If one claims to be a Christian but lives in known sin, thinking God's grace will cover them, they have another thing coming. God's grace is NOT a license to sin, but rather gives one the OPPORTUNITY TO REPENT. Jesus Christ came to save us from our sins and to TAKE OUR SINS AWAY
John 1:29 "The next day John seeth Jesus coming unto him, and saith, Behold the Lamb of God, which TAKETH AWAY the sin of the world."
Jesus Christ is not going to take away the guilt of our sins while we're still holding on to them.
It's also a bad idea to say "I'll sin now and repent later." That is presumption. We as believers are called to live holy lives acceptable to God.
3. Become unfruitful- The Bible says that if you don't abide in Jesus Christ and stay fruitful you will be cut off.
John 15:1-2 "I am the true vine, and my Father is the husbandman. Every branch in me that BEARETH NOT FRUIT HE TAKETH AWAY: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit."
John 15:6 "If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned."
No, I am not saying that we are saved by good deeds. The Bible makes it clear that it is by grace through faith we are initially saved. However, a genuine faith will always result in good works, especially in the area of evangelism.
Remember the parable of the servants and the talents, where the guys with 5 and 2 talents went out and made profit, while the guy with 1 talent was afraid and lazy and buried his talent? The guys that made good on their talents were accepted by the Master, whereas the guy with 1 talent had his talent taken away and cast into outer darkness. The lazy servant was even told that the very least he could have done is to put the talent in the bank so that the master could have gotten interest.
Now, realize that he might not have gotten a lot from the interest(banks never do give you much), but it is my theory that if the Lazy servant would have put the money into the bank that the master would have had mercy and not have cast him into the fire.
That leads to my point. We all need to be doing SOMETHING to serve the Lord and bring forth fruit. Even wearing a good Christian T-shirt(I don't like those Kerusso popular culture parodies, such as "Faithbook" and things, I like ordering custom shirts from Cafepress.com and Zazzle.com that have strong repentance messages on them that mention Hell and Repentance) is better than nothing. Putting some Christian bumper stickers on your car is another way to further the kingdom(yes it may lower the value on your car but if it leads to someone getting saved then who cares about a few hundred to a few thousand dollars).
There are numerous ways to spread the gospel, even if you have a timid personality like I do.
4. This is a special case, because I believe in a future rapture of true Christians and a 7 year tribulation period, but here it is:
If a Christian during the Tribulation Period was to take the Antichrist's 666 Mark in order to be able to buy or sell. Revelation 14:9-11 makes it very clear that ANYONE who takes the 666 mark will spend eternity in the Lake of Fire. That would include former Christians. Note that taking this mark is an unforgivable sin since you will be pledging your loyalty, trust, and worship to the Antichrist instead of to God, who ALONE deserves our loyalty, trust, and worship.
That will be a time of tremendous testing of one's loyalties to the uttermost, where God will be seeing if you truly love Him with all your heart or if you're really only loyal to yourself, as will be evidenced by your choice to either refuse the Mark, repent of your sins and submit to Jesus Christ, and risk starvation(which will be the result of not being able to buy and sell), intense persecution, rejection by society and even your former family and friends who take the mark, imprisonment, possible torture, and beheading.
If one was to relapse in their faith and take the mark, regardless of the circumstances, their eternal doom would be SEALED.