Can we lose our salvation?
This one is easy-- the Bible says you can.
"For in the case of those who have once been enlightened and have tasted of the heavenly gift and have been made partakers of the Holy Spirit, and have tasted the good word of God and the powers of the age to come, and then have fallen away, it is impossible to renew them again to repentance, since they again crucify to themselves the Son of God and put Him to open shame." Hebrews 6:4-6
For if we continue sinning willfully after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, but a terrifying expectation of judgment and THE FURY OF A FIRE WHICH WILL CONSUME THE ADVERSARIES. Anyone who has set aside the Law of Moses dies without mercy on the testimony of two or three witnesses. How much severer punishment do you think he will deserve who has trampled under foot the Son of God, and has regarded as unclean the blood of the covenant by which he was sanctified, and has insulted the Spirit of grace? For we know Him who said, "VENGEANCE IS MINE, I WILL REPAY." And again, "THE LORD WILL JUDGE HIS PEOPLE." It is a terrifying thing to fall into the hands of the living God.Hebrews 10:26-31
Hebrews 6 describes a Christian-- one who has tasted of the heavenly gift and has shared in the Holy Spirit-- and then falls away (which means no longer believes) then they cannot be turned back to repentance again.
Hebrews 10 talks about a believer who refuses to repent from their sin and the terrible punishment they will receive.
Theologically, this is also easy to understand. The basic requirement for receiving salvation is faith, right? Then if you no longer have faith, even if you once did, then you no longer have the salvation. Jesus uses a number of descriptions of those who refuse to obey Him or who refuse to believe in him and they will be burned in fire, cast out of the kingdom. This isn't just a temporary punishment, but his descriptions of eternal hell. You can look it up-- John 15:6; Matthew 7:21-27; John 3:36.
The reasons many say that there is no possibility of losing one salvation is twofold:
a. Theologically, people misunderstand the idea of predestination. Paul speaks of a church being predestined before God. But Paul is speaking of a group of people, not each one individually. It was predestined-- prophesied, in fact-- that the Gentiles would be saved. It was predestined that people of every nation would enter as citizens of God's kingdom. But Paul never said to Timothy, "You are predestined, nothing you say or do will send you to hell." Predestination is the salvation of a remnant, not of an individual.
b. Biblically, people misunderstand the passages of assurance. For instance, Romans 8, where Paul says, "Nothing can separate us from the love of God in Christ." But Paul was not speaking of our refusal to accept that love, to accept the salvation He is offering us. He was speaking of angelic powers-- demons-- and sufferings that people would claim that we do not have God's salvation if we suffer like that. Well, Paul says that sufferings or demonic forces cannot put a block between us and God. That is true. But if we refuse God's gift of obedience, His gift of the Holy Spirit, then He will not force us to accept salvation, even if we wanted it earlier in our lives.
Again, the argument against loss of salvation is picking up certain Scriptures, misunderstanding them, and ignoring others.