Hebrews 3:8

Saying that it is necessary to hope for good things from Christ, he now says that we must not harden our hearts towards this faith and hope, so that we do not suffer the same things as those who, in the desert, became unbelievers. And …

Paul, having treated of hope, and having said that "We are His house, if we hold fast the confidence and the rejoicing of the hope firm unto the end"; next shows that we ought to look forward with firmness, and he proves this from the Scriptures. His discourse was concerning Hope, and that it behooves us to hope for the things to come, and that for those who have toiled here there will assuredly be some reward and fruit and refreshment. This then he shows from the prophet. He says that there are "three" rests: one, that of the Sabbath, in which God rested from His works; the second, that of Palestine, into which when the Jews had entered they would be at rest from their hardships and labors; the third, that which is Rest indeed, the kingdom of Heaven; which those who obtain, do indeed rest from their labors and troubles. Of these three then he makes mention here. And why did he mention the three, when he is treating of the one only? That he might show that the prophet is speaking concerning this one. For he did not speak (he says) concerning the first. For how could he, when that had taken place long before? Nor yet again concerning the second, that in Palestine. For how could he? For he says, "They shall not enter into My rest." It remains therefore that it is this third.

But it is necessary also to unfold the history, to make the argument more clear. For when they had come forth out of Egypt, and had accomplished a long journey, and had received innumerable proofs of the power of God, both in Egypt, and in the Red Sea, and in the wilderness, they determined to send spies to search out the nature of the land; and these went and returned, admiring indeed the country, and saying that it abounded in noble fruits, nevertheless it was a country of strong and invincible men: and the ungrateful and senseless Jews, when they ought to have called to mind the former blessings of God, and how when they were hemmed in the midst of the armies of so many Egyptians, He rescued them from their perils, and made them masters of their enemies' spoils; and again, in the wilderness He clave the rock, and bestowed on them abundance of waters, and gave them the manna, and the other wonderful things which He wrought; when they ought, I say, to have remembered this, and to have trusted in God, they considered none of these things, but being struck with terror, just as if nothing had been done, they said, we wish to go back again into Egypt, "for God hath brought us out thither" to "slay us, with our children and wives." God therefore being angry that they had so quickly cast off the memory of what had been done, sware that generation, which had said these things, should not enter into the Rest; and they all perished in the wilderness. When David then, he says, speaking at a later period, and after these events, after that generation of men, said, "To-day, if ye will hear His voice, harden not your hearts," that ye may not suffer the same things which your forefathers did, and be deprived of the Rest; he evidently said this as of some future rest. […]

Having spoken about hope and about the need to await with confidence the reward and rest there from the labors here, he next proves on the basis of the prophet's words: believers will enter into rest, but unbelievers will not enter, just as the ancients …

What he cautions them against--hardening their hearts, turning the deaf ear to the calls and counsels of Christ: "When he tells you of the evil of sin, the excellency of holiness, the necessity of receiving him by faith as your Saviour, do not shut your ear and heart against such a voice as this." Observe, The hardening of our hearts is the spring of all our other sins.