Actually I’ve met a couple that believe a person can at one time be a believer then later curse God, kill Christians, and worship Satan and because they once believed they will be saved even without repenting. No matter what verses I put in front of them they insist that John 10:28 is proof someone cannot believe and lose their salvation no matter what they do.
Often in these discussions can be 3, 4, or more major theological questions, and another 3, 5, 7 minor ones, a good dozen or more, and often people can know several of them correctly, but need to look further at 1. This is good to remember because otherwise we make the mistake of thinking if they have a different view on 1 it means they are wrong on everything, while in fact they have some unique set of views, often many correct. In other words, just because we found someone last year, or last week, that was wrong on point after point, does not mean the person we are talking with now is also wrong on everything, of course.
Let's look at that John 10, v 28 meaning more humbly, fully --
25 Jesus answered them, “I told you, and you do not believe. The works that I do in my Father’s name bear witness about me,
26 but you do not believe because you are not among my sheep.
27 My sheep hear my voice, and I know them, and they follow me.
28 I give them eternal life, and they will never perish, and no one will snatch them out of my hand.
29 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.
30 I and the Father are one.”
We are protected from the powers of darkness, and no power on Earth, under Heaven, not even death, nor any other power can separate us from God's love, nor snatch us from His hand, but....still Christ warns us that our faith will falter and fail if we ourselves do not follow Him by doing as He said to do (Matthew 7:24-27), meaning while no other power can make us fall, we ourselves can fall, and then we need to turn, confess, repent, and return to Him, lest we remain "dead" even though we have already been a son (Luke chapter 15, the Prodigal, already a son, was departed and in the condition of being "dead", but finally repented, returned, confessed, and was forgiven therefore, and so was restored, and thus was "alive".)
No other power can take us from Him, but we ourselves can refuse to walk in the Spirit, and become more and more lost, needing to repent, and turn, and be restored.
What keeps us following Him? One thing He just said directly to you and me in those very 2 verses -- 27, 28 -- is that we listen to Him and follow Him.
If we are listening to Him, His words, and following what He said to do, then we can be assured no power can take us from His salvation of us.
It's just like John chapter 15 -- by remaining in His Words we can remain on the vine and bearing fruit, and not be cast into the fire : as branches already on the vine! can be cast into the fire if they bear no fruit -- John 15.