I'll take a swipe at this one, not doctrinally (you guys seem to have covered that quite well) but with my own personal testimony of the grace and mercy of Almighty God.
I met Christ at 23. My conversion was quite dramatic. Picture the worst person you can imagine, and then imagine him completely changed overnight, that was me.
Christ did a remarkable work in my life. I was so grateful, and everyone I met heard about it, everyone. All I ever talked about was what the Lord had done for me. I was zealous for the Gospel and zealous for good works. I found a great church and was very active in the ministry. This continued for probably a couple of years. Unfortunately my church turned out not to be as great as I thought. I won't bore you with the details, but they did me great harm, which led to a series of cascading events culminating in my leaving the church, and eventually the break up of my family.
Once it finally hit me what I had done, and the consequences of my actions, and it didn't take long, I was overwhelmed with regret and sorrow. Over the years I slipped deeper into sin. In fact, as sinful as I was when I met Christ, I had became even worse. This continued for just over three decades.
Alcohol, drugs, sexual immorality, anger, depression, multiple failed marriages, idolatry, indecency, spiritual deception, you get the idea. Through all of these struggles, I never denied Christ, I still shared the Gospel on occasion, I tried to be an honest person and a good dad to my kids. But I absolutely was not walking with Christ. I held on to my faith somewhat, but I did not study, did not pray, and though I tried many times, found no place for repentance. The bottom line is, my sins were over my head, I could not approach God. My guilt and shame made it practically impossible. I desired a relationship with Christ, I just couldn't make it happen. The something worse that Christ spoke of, had become my reality.
I did so much damage, and mainly to those I cared about the most. It's hard not to share this without weeping. But that is a recent development. I've never been a crybaby, that all changed about 18 months ago. My wife at that time wrecked her truck and almost died, a couple months later she left me. I thought at the time that He spared her life because she was not ready to meet him. Eventually I concluded that He spared her life because I was not ready to meet Him. If she had died, I would have grieved somewhat (my marriage was hell) and then replaced her with another just like her, dangerous women had been a pattern in my life. But she left me instead, and I needed answers. So after 30 years of running away, I turned back to Christ. I had no where else to go.
Please bare with me, I don't know how to tell this story briefly.
In the 30 years that I lived as the prodigal son, I had only really prayed 3 times. Those of you who know what real prayer is will understand this. The first time was after my grandfather died and my grandmother was in the hospital soon to join him. I called out to God for a very long time. I questioned His love for me, I questioned my salvation, I wanted answers. I got none. After what might have been an hour or more of exhausting prayer, in sorrowful silence upon my knees, I heard these words spoken within my heart, "My grace is sufficient for you."
I cried out in reply, Is that all you've got to say to me?
More silence, apparently it was. Nothing changed for me that day, but I never forgot what had been spoken to me, I never will.
Many years later I was at work and I heard the Lord speak again to my heart, "Pray for your son." I knew it was the Lord, and I immediately began to pray for my son. I didn't know why or even what I was praying over, so I prayed for his life and safety. I found out a few days later that he had been attacked and brutally beaten and almost died at the precise hour that I was calling out to God on his behalf. I was grateful and even hopeful, but again, nothing changed in my life.
Moving forward another several years, maybe five, I was again at work and had stopped (I'm a professional driver) to take a quick break. I reached back into my bunk for a bottle of water when it was as if someone grabbed me by the wrist and pulled me to my knees. I began to cry out to God in a way I had maybe never done before. I confessed my many sins for what seemed like a very long time. I wept before His throne until I could weep and pray and confess no more. I said to the Lord, "I don't know what else I could possibly confess. The Lord said to me, " Give me a minute." That minute turned into another half hour or so, in which God revealed to me not only the depth, but the breadth of my transgressions against Him.
Sin is deceitful my brothers and sisters, make your calling and election sure.
This time, I had changed. In fact, I hadn't been so full of joy and hope since first knowing Him so many years before. But this new found hope was short lived. I called my wife who is supposed to be a believer herself to tell her all about it, my daughter too. But they couldn't relate. And my wife began to undermine my newfound repentance the moment I arrived at home. And she succeeded.
I slipped back into my old ways and the next two years leading up to her wreck were living hell.
Which brings me to the present. "To see ones own sins, and to weep over them, the man who has this blessing has everything." I don't know who made this statement, but I can attest to it's truthfulness. Unless we are able to see the fullness of our sin, we cannot experience the fullness of Gods grace. Those who are forgiven much, love much. I'm walking with Christ again, it's not a perfect walk, far from it. But Christ's love for me is a perfect love, and His grace is truly sufficient.
I will probably never shake the feelings of regret for the choices that I made, the sorrow over the sins that I have sinned. Nor could I count the tears I've cried in this last year over these sins or the prodigal (wasteful) life I've lived. But neither could I number the tears I've cried, often simultaneously, as I behold Christ at the right hand of the Father interceding on my behalf.
In summation. If you had met me at anytime during this long period of my prodigal wanderings, you may have judged me unfit, unredeemed and unworthy, and I could not fault you for doing so. But you would have been terribly mistaken.
For every scripture posted in this thread about the dangers of falling away, I could point you to a scripture about the wonders of Gods grace.
To my backslidden brothers and sisters who may be wondering and worrying if maybe they have sinned too big, for too long, or wandered too far from the grace and mercy of Almighty God, I refer you to Psalm 139.
Christ is your salvation. Christ is your sanctification.
Christ is your Good Shepherd, and the Lover of Your Soul.
The Spirit of God is the Spirit of Christ, you cannot flee from the Spirit of God. Even if it feels like you are in the very depths of hell. Christ is there. He has never left you, He will never forsake you. He will not lose any whom the Father has given to Him. You are safe, you are loved, you are seated with Him at the right hand of the Father.
For those of you who are merely seekers and have not yet known the joy of the Lord, this is the love that you've been searching for. Surrender to His love. Harden not your hearts. Receive the free gifts of repentance and faith, and commit yourself to understanding and doing His will. And you will be eternally saved, eternally secure. Diligently seek His face, and you will be greatly rewarded for doing so, in this life and in the next.
So, no. You cannot lose your salvation. Christ is the Author and the Finisher of your faith. Christ is your Salvation. We are saved by grace (which leads to repentance) through faith (which leads us to Christ), not of yourself, but this is the gift of God.