Both verses are 100% true.
But who are the type of sheep that cannot be plucked out of His hand?
The passage says it is the sheep that FOLLOW Jesus. These are not rebellious sheep being dragged about on leashes by their necks.
And Hebrews 6:4-6 is talking about apostasy after having the Spirit and the gifts thereof. It is talking about a denial of Jesus after having the Holy Spirit. It is not talking about backsliding into sin (Which is forgivable according to James 5:19-20). Anyways, we know for a fact that a believer can commit apostasy because the writer of Hebrews in verse 9 says he is convinced that such is not the case for those he is writing to here.
I think Hebrews is describing Blasphemy against the Holy Spirit, which is unforgivable. John says; "no man is able to pluck them out of my Father's hand". While Hebrews implies that we ourselves can turn away and hold him in contempt. jmo
And that is EXACTLY what is being said in these two passages. As long as we maintain an honest and true faith, as evidenced by our fruit, there is absolutely nothing that can or will part us from God. It is ONLY by renouncing one's faith that we can lose that assurance. And if it were not possible to renounce one's faith, it wouldn't even be brought up as a hypothetical, there'd be no need for the new converts to be encouraged to continue and warned about slacking.
And lest anyone say that this reflects a works-based salvation, that's not true as the following verses clearly state that salvation is based on faith and faith (or the loss thereof) is not an act of works:
Romans 4:
1 What then shall we say that Abraham, our forefather according to the flesh, discovered in this matter?
2If, in fact, Abraham was justified by works, he had something to boast about—but not before God.
3 What does Scripture say? “Abraham believed God, and it was credited to him as righteousness. [SUP]
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4Now to the one who works, wages are not credited as a gift but as an obligation.
5However, to the one who does not work but trusts God who justifies the ungodly, their faith is credited as righteousness.
6David says the same thing when he speaks of the blessedness of the one to whom God credits righteousness apart from works
13It was not through the law that Abraham and his offspring received the promise that he would be heir of the world, but through the righteousness that comes by faith.
14For if those who depend on the law are heirs, faith means nothing and the promise is worthless,
15because the law brings wrath. And where there is no law there is no transgression.
16Therefore, the promise comes by faith, so that it may be by grace and may be guaranteed to all Abraham’s offspring—not only to those who are of the law but also to those who have the faith of Abraham. He is the father of us all.
17As it is written: “I have made you a father of many nations.” He is our father in the sight of God, in whom he believed—the God who gives life to the dead and calls into being things that were not.
18Against all hope, Abraham in hope believed and so became the father of many nations, just as it had been said to him, “So shall your offspring be.”[SUP]
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19Without weakening in his faith, he faced the fact that his body was as good as dead—since he was about a hundred years old—and that Sarah’s womb was also dead.
20Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God,
21being fully persuaded that God had power to do what he had promised.
22This is why “it was credited to him as righteousness.”
23The words “it was credited to him” were written not for him alone,
24but also for us, to whom God will credit righteousness—for us who believe in him who raised Jesus our Lord from the dead.
Eph 2:8-9 For by grace are you saved through faith, and that not of yourselves. It’s a gift of God and not of works, let any man should boast.
Acts 16:
31 They replied, “Believe in the Lord Jesus, and you will be saved”
"Therefore, since we have been justified by faith, we have peace with God through our Lord Jesus Christ." Romans 5:1
John 5:24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
Romans 11:6 And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work.