Hebrews 6:4

Remember to whom this epistle is written:
The book of Hebrews was written for the Jews who had accepted Jesus as their Messiah. They were in danger of slipping back into the traditions of Judaism because they had not put down roots in the soil of Christianity.
The apostle was addressing Christians. He was endeavoring to keep them from apostasy. The object was not to keep those who were awakened and enlightened from apostasy, but it was to preserve those who were already in the Church of Christ, from going back to Judiasm ( Dead Works ).
Chapter 6:4 and thus they render their salvation impossible, by wilfully and maliciously rejecting the Lord that bought them.
Observation: 6:4 Notice it says it is impossible for those who were ONCE enlightened, not those who continue to be enlightened. Enlightened of what? Of the principals and doctrine of Christ v 1
Interpretation: Men who reject the truth of God, who once believed all the doctrine of God, are termed apostates ( A person who renounces a religion or faith ).
God does not Indian give salvation:
Joh 10:27 My sheep respond to my voice, and I know who they are. They follow me,
Joh 10:28 and I give them eternal life. They will never be lost, and no one will tear them away from me.
Joh 10:29 My Father, who gave them to me, is greater than everyone else, and no one can tear them away from my Father.
Mat 7:21 "Not everyone who says to me, 'Lord, Lord!' will enter the kingdom of heaven, but only the person who does what my Father in heaven wants.
Mat 7:22 Many will say to me on that day, 'Lord, Lord, didn't we prophesy in your name? Didn't we force out demons and do many miracles by the power and authority of your name?'
Mat 7:23 Then I will tell them publicly, 'I've never known you. Get away from me, you evil people.'
It is not the doing of "things", however spiritual they may be, that gains us entrance into heaven. Only the person who does God's will will gain entrance into the kingdom of heaven.
1Jn 5:3 For this is the love of God, that we keep His commandments, and His commandments are not burdensome.
Rom 8:14 For as many as are led by the Spirit of God, they are the sons of God.
These Apostates spoken of in v4 had the knowledge of who Christ was, tasted the good word of God ( Salvation in Jesus and Him alone), and fell away ( apostacy ). Since having been enlightened, the apostate takes back his alegenance. See 2nd Peter Chapter 2
The doctrine of the Person and the glory of Jesus forms the subject of revelation in the epistle, and was the means of deliverance for the Jews from the whole system which had been such a heavy burden on their hearts; it should prevent their forsaking the state described in Verses 4 and 5.
Observation: 6:6 If a person who was once enlightened, turns and rejects Jesus outright, he is an apostate and cannot be brought back to the faith that they reject. It is impossible to call an apostate a believer, or a follower of Christ an Apostate.
Was not Judas called a disciple?
Joh 6:65 And He said, Because of this I said to you that no one can come to Me unless it was given to him from My Father.
Joh 6:66 From this time many of His disciples went back into the things behind, and walked no more with Him.
Joh 6:67 Then Jesus said to the Twelve, Do you also wish to go away?
Joh 6:68 Then Simon Peter answered Him, Lord, to whom shall we go? You have the Words of eternal life.
Joh 6:69 And we have believed and have known that You are the Christ, the Son of the living God.
Joh 6:70 Jesus answered them, Have I not chosen you, the Twelve? And one of you is a devil?
Joh 6:71 But he spoke of Judas Iscariot, the son of Simon; for it was he who was about to betray Him, being one of the Twelve.
Which disciple am I? God forbid I should be the disciple that went back into the things prior to Christ v 66
Looking at the scope of the whole passage, it appears to us that the Apostle wished to push the disciples on. There is a tendency in the human mind to stop short of the high calling of God for aour lives. As soon as ever we have attained to the first principles of religion, have passed through baptism, and understand the resurrection of the dead, there is a tendency in us to sit still; to say, "I have passed from death unto life; here I may take my stand and rest;" whereas, the Christian life was intended not to be a sitting still, but a race, a perpetual motion. The Apostle, therefore endeavours to urge the disciples forward, and make them run with diligence the heavenly race, looking unto Jesus. He tells them that it is not enough to have on a certain day, passed through a glorious change—to have experienced at a certain time, a wonderful operation of the Spirit; but he teaches them it is absolutely necessary that they should have the Spirit all their lives—that they should, as long as they live, be progressing in the truth of God. In order to make them persevere, if possible, he shows them that if they do not, they must, most certainly be lost; for there is no other salvation but that which God has already bestowed on them, and if that does not keep them, carry them forward, and present them spotless before God, there cannot be any other. For it is impossible, he says, if ye be once enlightened, and then fall away, that ye should ever be renewed again unto repentance.
Final Perseverance
A Sermon (No. 75) Delivered on Sabbath Morning, March 23, 1856, by the REV. C.H. SPURGEON At New Park Street Chapel, Southwark