The following is an excerpt from The MacArthur New Testament Commentary on Hebrews 6.

Because they believe the warning is addressed to Christians, many interpreters hold that the passage teaches that salvation can be lost. If this interpretation were true, however, the passage would also teach that, once lost, salvation could never be regained. If, after being saved, a person lost his salvation, he would be damned forever. There would be no going back and forth, in and out of grace. But Christians are not being addressed, and it is the opportunity for receiving salvation, not salvation itself, that can be lost.

The believer need never fear he will lose his salvation. He cannot. The Bible is absolutely clear about that. Jesus said, “My sheep hear My voice, and I know them, and they follow Me; and I give eternal life to them, and they shall never perish; and no one shall snatch them out of My hand. 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” (John 10:27–29). Paul is equally clear. “Who shall separate us from the love of Christ? Shall tribulation, or distress, or persecution, or famine, or nakedness, or peril, or sword? … For I am convinced that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, nor height, nor depth, nor any other created thing, shall be able to separate us from the love of God, which is in Christ Jesus our Lord” (Rom. 8:35, 38–39). “He who began a good work in you will perfect it until the day of Christ Jesus” (Phil. 1:6). We are “to obtain an inheritance which is imperishable and undefiled and will not fade away, reserved in heaven,” and we “are protected by the power of God through faith for a salvation ready to be revealed in the last time” (1 Pet. 1:4–5). If the power of God cannot keep us, nothing is dependable or trustworthy or worth believing in. A Christian has no reason at any point in his life to believe that his salvation is or can be lost. If by Christ’s death we can be saved, certainly by His life of power and intercession we can be kept saved (Rom. 5:10).

It is unbelievers who are in danger of losing salvation—in the sense of losing the opportunity ever to receive it. The unbelieving Jews were in great danger, because of their spiritual immaturity and sluggishness, of turning back to Judaism and of never being able to repent and come to Christ. They would be lost forever, because they had rejected, at the most vital point in knowledge and conviction, the only gospel that could save them. There is no other salvation message they could hear, no evidence of the truth of the gospel they had not seen.

These particular Jews had even heard the apostles preach and had seen them perform signs and wonders and miracles (Heb. 2:4). They had been privileged to behold virtually all the manifestations of His saving Word and power that God could give. They had heard it all and seen it all. They even had accepted it all intellectually. Any who are so informed, so witnessed to, so blessed with every opportunity to know God’s gospel, and who then turn their backs on it—for Judaism or anything else—are eternally lost. They not only reject the gospel, but crucify to themselves the Son of God, and put Him to open shame. They had either to go on to full knowledge of God through faith in Christ or else turn away from Him, to become apostate and be lost forever. There was no other alternative.