Since you admit that there are many passages which by themselves can be used to defend opposite positions on this question, what makes you think salvation can’t be lost?
Here are some things to consider:
If salvation can be lost, this requires a reversal of regeneration. This means the born again must become unborn again. And if they subsequently repent, then they must become born again again. Can a man also be born again again again? Where does it end? Is regeneration really so transient?
If salvation can be lost, “eternal life” cannot be eternal life. It could only be temporary hope for eternal life that couldn’t become eternal life until after we die. As long as we’re still here, how can we call “eternal” what can be lost? Scripture says believers currently have eternal life, not that we might eventually have eternal life. It says that the life we have is eternal, not possibly or potentially eternal (or “to be determined” by whether we manage to keep or lose it). It says we can know that we have eternal life (1 John 5:13). How can we know that if our salvation isn’t secure?
If we can lose our salvation, we can also keep it—by abstaining from whatever it takes to lose it. This makes it at least partially dependent on our merits. This flies in the face of Scripture’s teaching that salvation is a miraculous work of God, dependent on Him, not us. If we can lose our salvation, how can we lose it? How can we keep it? What does our answer say about the nature of salvation and who it depends on? Is salvation a 50/50 proposition? Is it 10% us and 90% God? 90% God and 10% us? (Rewards are earned by our work for God, done in dependence on him, but salvation is not earned, it’s not a reward-it’s a gift, earned only by Christ, not us.)
Question: If Judas had died soon after he followed Christ would he have gone to Heaven or Hell? If Peter had died soon after he denied Christ, would he have gone to Heaven or Hell? What do our answers suggest about what we really believe concerning the nature of salvation?
If salvation is the work of Christ, and we can lose our salvation, wouldn’t this constitute an ultimate failure on Christ’s part? Jesus said, “My sheep hear my voice, and I know them, and they follow me. I give them eternal life, and they will never perish, and no one will snatch them out of my hand. My Father, who has given them to me, is greater than all, and no one is able to snatch them out of the Father’s hand” (John 10:27-29). What could we expect Jesus to say to make it more emphatic that if someone is truly saved they cannot lose their salvation?
For more information on this subject, see Randy Alcorn's book Heaven.