Some believers struggle with assurance. Assurance is the inner knowledge that we are, for sure, right with God; that our sins are for sure forgiven; that we are certainly on our way to heaven.
Assurance can be connected to personality as well as doctrinal teaching. People with low self-esteem and confidence can also suffer from a lack of salvation-assurance.
Believers from theologically "Arminian" backgrounds can also suffer from assurance.
(In simple terms, "Arminianism" says that salvation is in our hands. One day I can believe and the next day I can decide not to believe. In contrast, "Calvinists" believe that salvation is completely of the Lord. God decides who is saved and God keeps them by his almighty power through all the struggles of this passing life.)
For sure, Satan uses any chink in our armour, whether doctrinal, personality - and most frequently failure - to "accuse" us. The name "Satan" means accuser. He says to us "How can you be a true believer when you just thought that, said that, did that?" And all too often we believe his lies.
The answer is a radical "NO"
So can a believer lose their salvation? Absolutely, and certainly "No!" Why not?
(i) God is the one who chooses who is saved. "Salvation is of the Lord" according to Scripture (Psalm 3:8, Jonah 2:9). If salvation had anything to do with us, we would have no hope, because outside of Christ we were lost in sin (Ephesians 2:1) and under the power of Satan (1 John 5:19). If salvation depends on us choosing it, not one person would ever do so. It is God who sovereignly chooses who is saved (Ephesians 1:4)
(ii) Those whom God chooses, he for sure brings to heaven. "He who began a good work... will carry it on to completion" (Phil 1:6). "I give them eternal life, and they shall never perish; no one will snatch them out of my hand." (John 10:28). Nothing can separate us from the love of God (Romans 8).
If salvation was in our hands, as some Arminians teach, the expereince of assurance is completely unattainable. But if salvation is in God's hands, we can have every confidence, in spite of all the ups and downs of life, that we are safe.
What about those who have walked out?
Someone says, "But what about those who once believed but don't any longer?" Don't they demonstrate that we can be lost?" Not at all. A "backslidder" is a true believer who leaves for a while but in the end returns. The prodigal son returned home; Peter returned to the Master after denying him. King David returned. If someone truly believed, but is now backslidden they will one day return. A backslidder is a true believer all the time.
An apostate is someone who looked like a true believer but never was and one day leaves for good and thereby demonstrates that fact. Judas was an apostate.
What about Hebrews 6?
The book of Hebrews was written to backslidden believers on the verge of walking out. How do we bring such back? With many encouraging truths and with the severest warnings, that if they walk out they may never ever return. That's what the warning of Hebrews 6 is about.
Won't assurance lead to spiritual apathy?
Again, no. Because the mark of a true believer is that they work out their salvation with fear and trembling (Philippians 2:12). They don't rest on their spiritual laurels saying to themselves, "I'm saved so I don't need to bother". That lazy attitude is almost certainly the mark of someone who is not saved.
Assured that we are loved by God and eternally saved should be the experience and blessing of all Christian.
Blessed assurance, Jesus is mine;
Oh, what a foretaste of glory divine!
Heir of salvation, purchase of God,
Born of His Spirit, washed in His blood.
This is my story, this is my song,
Praising my Savior all the day long.
This is my story, this is my song,
Praising my Savior all the day long