These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God.
Here John tells us that eternal life was given to us (believers) and that we can know that we are saved, that we have eternal life. And notice the tense there… we "have" eternal life. Not we "will have", but we "have" eternal life. It's ours. Right now.
Question: How could anyone possibly know that they are saved if there's a possibility that they might lose their salvation?
We have (or ought to have) absolute 100% assurance of our salvation in Jesus Christ. There is no reason to have anything less than 100% assurance. Christ has promised eternal life for those who place their trust in Him. So, is Christ trustworthy or not? If you place your trust in Christ for your salvation, then you have eternal life. It's as simple as that. And once again… how long does eternal life last? Forever.
Since Christ is 100% trustworthy, since we can be absolutely certain that His promises will be kept, what reason could we possibly have to doubt our salvation apart from a reluctance to actually place our trust in Christ?
What about the person who believes that salvation is something that can be lost due to sin, etc. but is nevertheless certain that they will be in Heaven? Who is this person trusting? Themselves, or Christ?
Back to the skydiving illustration again. Essentially, the Bible says to jump out of the plane with no reserve 'chute. Christ is all you need. His parachute will save you. There is absolutely zero possibility that His parachute will fail to open properly, tear, get tangled during deployment or any other malfunction you can imagine. Don't pack the reserve, you don't need it and it won't do you a bit of good. Rely on Christ alone and know that you have eternal life.