And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father’s hand.
If we could lose our salvation… or if we could give it back or willingly forfeit our salvation, then we would, in fact, be able to pluck ourselves out of the Father's hand. John doesn't seem too equivocal here when he says "no man is able." He doesn't say "most men, but you might be able to if you try hard enough." No. He says no man, and of course he means human here, so it's not as though women might be able to while men can't. He, he.
It's very, very difficult to imagine a way to escape such clear verses. What could Christ have meant other than to say "Folks, my grace is so complete and perfect that once you've placed your trust in me, you have eternal life right, then and there and you will be in Heaven with me someday and there is nothing you can do to change that."
We are saved by grace. It is a gift. That means it costs us nothing. And if we think we can lose our salvation, then we must also think that we are doing something to keep it. Christ tells us not to trust ourselves for our salvation. He says to trust in Him. He says not to trust in our obedience, because we will fail. Christ did it all and He keeps us secure whether we deserve it or not. (and that's certainly a "not") That's grace.