It is not a question of whether a believer can lose his salvation. Once a person is sealed with the Holy Spirit, it is irrevocable:
In Him you also, when you heard the word of truth, the gospel of your salvation, and believed in Him, were sealed with the promised Holy Spirit, who is the guarantee of our inheritance until we acquire possession of it, to the praise of His glory. (Ephesians 1:13-14)
But the same apostle who wrote that comforting passage also passionately expressed this desire:
...that I may know Him and the power of His resurrection, and may share His sufferings, becoming like Him in His death, that by any means possible I may attain the resurrection from the dead. Not that I have already obtained this or am already perfect, but I press on to make it my own, because Christ Jesus has made me His own. (Philippians 3:10-12)
In the Apostle Paul's mind, there was a goal that required tremendous effort on his part to reach, and to which he had not yet attained, and which he could fail to attain. What was the resurrection he had to strive for? Are not all believers resurrected at Messiah's coming, unless they are alive when He returns?
Then I saw thrones, and seated on them were those to whom the authority to judge was committed. Also I saw the souls of those who had been beheaded for the testimony of Jesus and for the word of God, and who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life and reigned with Christ for a thousand years. (The rest of the dead did not come to life until the thousand years were ended.) This is the first resurrection. Blessed and holy is the one who shares in the first resurrection! Over such the second death has no power, but they will be priests of God and of Christ, and they will reign with Him for a thousand years. (Revelation 20:4-6)
Clearly, Paul understood that reigning with Christ during His millennial kingdom on the earth was not a foregone conclusion for a believer, but depended on how one lived his life in this age. It was possible to be among "the rest of the dead" who did not come to life until the thousand years were ended. This possibility of being disqualified to rule with Him is mentioned in many places in the New Testament. For example:
And you, who once were alienated and hostile in mind, doing evil deeds, He has now reconciled in His body of flesh by His death, in order to present you holy and blameless and above reproach before Him, if indeed you continue in the faith, stable and steadfast, not shifting from the hope of the gospel that you heard, which has been proclaimed in all creation under heaven, and of which I, Paul, became a minister. (Colossians 1:21-23)
What if you don't continue in the faith, stable and steadfast? Will you still be blameless and above reproach before Him?
He who has an ear, let him hear what the Spirit says to the churches. He who overcomes shall not be hurt by the second death. (Revelation 2:11)
What if you don't overcome? Will you be "hurt" by the second death? What does it mean to be "hurt" by the second death?
You have a few names even in Sardis who have not defiled their garments; and they shall walk with Me in white, for they are worthy. He who overcomes shall be clothed in white garments, and I will not blot out his name from the Book of Life; but I will confess his name before My Father and before His angels. (Revelation 3:4-5)
What if you have defiled your garments? Will you still walk with Him in white? Will you still be worthy? If you don't overcome, will your name remain in the Book of Life -- the register of those who are counted worthy to rule with Him?
So Christ, having been offered once to bear the sins of many, will appear a second time, not to bear sin, but to save those who are eagerly waiting for Him. (Hebrews 9:28)
What if you are not eagerly waiting for Him?
For, "Yet a little while, and the coming One will come and will not delay; but My righteous one shall live by faith, and if he shrinks back, My soul has no pleasure in him." (Hebrews 10:37-38)
Evidently it is possible for one of His righteous ones (to whom He imputed His righteousness when they first believed) to be among those who shrink back when He returns. Let's back up a few verses to see how this is possible:
For you have need of endurance, so that when you have done the will of God you may receive what is promised. (Verse 36)
What if you don't do the will of God? What if you lack endurance? Will you still receive what is promised?
Therefore do not throw away your confidence, which has a great reward.
How can you throw away your confidence? And if you do, what will be your reward?
For we know Him who said, "Vengeance is mine; I will repay." And again, "The Lord will judge His people." It is a fearful thing to fall into the hands of the living God. (Verses 30-31)
Who are "His people" who are judged and fall into the hands of the living God?
How much worse punishment, do you think, will be deserved by the one who has spurned the Son of God, and has profaned the blood of the covenant by which he was sanctified, and has outraged the Spirit of grace? (Verse 29)
How does one spurn the Son of God or profane the blood of the covenant or outrage the Spirit of grace? Is this talking about believers?
For if we go on sinning deliberately after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a fearful expectation of judgment... (Verses 26-27)
Have you received the knowledge of the truth? What if you do go on sinning deliberately?
For it is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, if they then fall away, since they are crucifying once again the Son of God to their own harm and holding Him up to public shame. (Hebrews 6:4-6)
What does it mean to "fall away" or to hold the Savior up to public shame? What becomes impossible? Will such a one be "hurt" by the second death? What is the second death? Did you know there are actually two distinct types of "second deaths" mentioned in the Scriptures? The second death for unbelievers we all know about -- eternity spent in the lake of fire.1 But the second death for disciples -- which will be for the foolish virgins only2 -- is to miss the millennial kingdom rule of Messiah on the earth. These foolish virgins will spend a thousand years in death with the unbelievers3 to pay for their unconfessed sins. They did not give themselves to purification in this age and so their souls were not saved from death.4 They are unable to be presented to their King as whole and complete and so will miss out on the kingdom age.5
Therefore putting aside all filthiness and all that remains of wickedness, in humility receive the word implanted, which is able to save your souls. (James 1:21)
What if you don't put aside all filthiness and wickedness? What if the standards of His word do not become part of your very nature, as expressed in your behavior? What will happen to your soul?
For this very reason, make every effort to supplement your faith with virtue, and virtue with knowledge, and knowledge with self-control, and self-control with steadfastness, and steadfastness with godliness, and godliness with brotherly affection, and brotherly affection with love. For if these qualities are yours and are increasing, they keep you from being ineffective or unfruitful in the knowledge of our Lord Jesus Christ. For whoever lacks these qualities is so nearsighted that he is blind, having forgotten that he was cleansed from his former sins. Therefore, brothers, be all the more diligent to make your calling and election sure, for if you practice these qualities you will never fall. For in this way there will be richly provided for you an entrance into the eternal6 kingdom of our Lord and Savior Jesus Christ. (2 Peter 1:5-11)
What if you don't add virtue7 and self-control to your faith? What if you don't diligently increase in steadfastness and godliness and love? Will entrance into the Kingdom still be richly provided to you?
Blessed is the man who remains steadfast under trial, for when he has stood the test he will receive the crown of life, which God has promised to those who love Him. (James 1:12)
What if you don't remain steadfast? Will you still receive the crown of life? If you don't stand the test, what is the proof that you love Him?
If you love Me, you will keep My commandments. Whoever has My commandments and keeps them, he it is who loves Me. And he who loves Me will be loved by My Father, and I will love him and reveal Myself to him. (John 14:15,21)
What if you don't keep His commandments? Do you still love Him? Will He still reveal Himself to you? Or will you be shown to be a liar?
Whoever says "I know Him" but does not keep His commandments is a liar, and the truth is not in him. (1 John 2:4)
Is the truth in you?
Truly, truly, I say to you, if anyone keeps My word, he will never see death. (John 8:51)
What if you don't keep His word? Will you see death?