The passage say:My sheep hear my voice, and I know them, and they follow me. I give them eternal life, andthey will never perish, andno one will snatch them out of my hand.In truth, this passage only proves, firstly, that eternal life is a gift for those who hear and follow and, as long as one continues to hear and follow he has eternal life and so of course he will not perish.  This passage does not say that one cannot choose to discard eternal life and therefore will perish.  Eternal life is a gift; not a penalty.  Once received, one cannot choose not to have a penalty.  However, one can choose not to have a gift after receiving it.  The gift of eternal life can be discarded by ceasing to hear and follow.  Secondly, this passage only proves that you cannot be snatched away from salvation; it does not say that you cannot walk away from salvation.  You walk away from salvation by choosing to follow heresy, or choosing not to endure persecution, or choosing to wilfully persist in sin.  However, as long as you hear and follow, you cannot be snatched away from salvation.