Thanks for presenting your passage and your point of emphasis as noted, specifically in verses 27-29. I would like to first acknowledge that this passage most definitely reveals a security found in Christ as revealed in these verses. So, from this standpoint I would wholeheartedly agree with you. We discover that our Lord has committed himself to keeping those who are his, revealing that no one is able to rip or take any of his sheep (children) away from him. This is security indeed!
Notwithstanding, our Lord has also qualified those who are the one’s who will benefit from these statements as found in verse 27. Three points of emphasis must be noted: 1)The sheep hear His voice – Identifying those who are receptive to His word; 2) I know them – Acknowledging the Lord’s omniscience and personal interest in His sheep; 3) And they follow me – Identifying man’s cooperative response to God’s grace unto salvation.
There are clear truths presented to those who would share in this eternal life as revealed in these verses. Christ conditioned the security of not being snatched away from him on the basis of the sheep (Christian), who both hear, and willfully follow him – meaning that this eternal security is found only on these grounds; hence, any sheep who do not hear and is not following him cannot claim this promise.
Finally, it must be also noted that the each of the verbs mentioned in verse 27 are present indicatives. The most constant characteristic of the [Greek] Present Indicative is that it denotes action in progress, which acknowledges the condition of ‘hearing and following’ as governing the promise of never perishing. This is also seen in John 8:51, declaring “Verily, verily, I say unto you, if a man keep my word, he shall never see death.” It is from this standpoint that the Scriptures consistently declare a conditional basis for an eternal security offered to the saints.