So we see - We see from the direct testimony of the Old Testament that unbelief was the reason why they were excluded from the promised land. Let us learn in view of the reasoning and exhortations here:
(1) The evil of unbelief. It excluded that whole generation, consisting of many hundred thousand souls, from the land of promise - the land to which they had looked with ardent hopes, and with warm desires. It will exclude countless millions from heaven. A "lack of confidence in God" is the great source of evil in this world, and will be the cause of wretchedness to all eternity of unnumbered hosts. But surely that was not a small or unimportant thing which strewed the desert with the bones of that whole generation whom God had in so remarkable a manner rescued from Egyptian servitude. And that cannot be a small matter which will cause multitudes to sink down to infinite wretchedness and despair.
(2) let us who are professed Christians be cautious against indulging unbelief in our hearts. Our difficulties all begin there. We lose confidence in God. We doubt his promises, his oaths, his threatenings. In dark and trying times we begin to have doubts about the wisdom of his dealings, and about his goodness. Unbelief once admitted into the heart is the beginning of many woes. When a man loses confidence in God, he is on a shoreless ocean that is full of whirlpools, and rocks, and quicksands, and where it is "impossible" to find a secure anchorage. There is nothing to which he may moor his driven bark; and he will never find safety or peace until he comes back to God.
(3) let us live a life of faith. Let us so live that we may say with Paul, "The life that I now live in the flesh I live by the faith of the Son of God, who loved me and gave himself for me." So living, we shall have peace. The mind will be at rest. Storms and tempests may blow, but we shall be secure. Others may be troubled in the vicissitudes of life, but our minds will be at peace.
(4) let us live expecting the future "rest" that remains for us. Let us keep our eye fixed upon it. To us there is a rest promised, as there was to the Hebrews whom God had delivered from the land of oppression; and we may by faith attain to that "rest" as they might have reached the land of Canaan.
(5) let us persevere to the end. He that draws back must be lost. He that does not endure to the end of life in the ways of religion can never have been a Christian. There is nothing which will furnish certain evidence of religion unless our piety is such as to lead us to persevere until death. The man who enters on the professed Christian life expecting to fall away, or who can look upon the possibility of falling away without concern, has never known anything of the nature of true religion. He cannot be a Christian. He may have had raptures and visions; he may be a loud professor and a noisy and zealous partisan, but he has no evidence that he has ever known anything about religion. That religion which is not connected with a firm and determined purpose by the grace of God to persevere to the end of life, is no true religion; and a man who expects to fall away and go back again to the world, or who can look at such an idea without alarm, should regard it as a settled matter that he has no true knowledge of God.
(6) no man should delay the work of salvation to a future time. today is the accepted time; today the only time of which we have any security. God speaks "today," and today his voice should be heard. No man on any subject should defer until tomorrow what ought to be done today. He who defers religion until a future time neglects his own best interest; violates most solemn obligations; and endangers his immortal soul. What security can anyone have that he will live to see another day? What evidence has he that he will be any more disposed to attend to his salvation then than he is now? What evidence can he have that he will not provoke God by this course, and bring condemnation on his soul? Of all delusions, that is the most wonderful by which dying people are led to defer attention to the concerns of the soul to a future period of life. Nowhere has Satan such advantage as in keeping this delusion before the mind; and if in respect to anything the voice of warning and alarm should be lifted loud and long, it is in reference to this. O why will not people be wise "today?" Why will they not embrace the offer of salvation "now?" Why will they not at once make sure of eternal happiness? And why, amidst the changes and trials of this life, will they not so secure the everlasting inheritance as to feel that that is safe - that there is one thing at least that cannot be shaken and disturbed by commercial embarrassment and distress; one thing secure though friends and kindred are torn away from them; one thing safe when their own health fails, and they lie down on the bed where they will bid adieu to all earthly comforts, and from which they will never rise?

So we see that they could not enter in - It was no decree of God that prevented them, it was no want of necessary strength to enable them, it was through no deficiency of Divine counsel to instruct them; all these they had in abundance: but they chose to sin, and would not believe. Unbelief produced disobedience, and disobedience produced hardness of heart and blindness of mind; and all these drew down the judgments of God, and wrath came upon them to the uttermost.
1. This whole chapter, as the epistle in general, reads a most awful lesson against backsliders, triflers, and loiterers in the way of salvation. Every believer in Christ is in danger of apostasy, while any remains of the evil heart of unbelief are found in him. God has promised to purify the heart; and the blood of Christ cleanses from all sin. It is therefore the highest wisdom of genuine Christians to look to God for the complete purification of their souls; this they cannot have too soon, and for this they cannot be too much in earnest.
2. No man should defer his salvation to any future time. If God speaks to-day, it is to-day that he should be heard and obeyed. To defer reconciliation to God to any future period, is the most reprehensible and destructive presumption. It supposes that God will indulge us in our sensual propensities, and cause his mercy to tarry for us till we have consummated our iniquitous purposes. It shows that we prefer, at least for the present, the devil to Christ, sin to holiness, and earth to heaven. And can we suppose that God will be thus mocked? Can we suppose that it can at all consistent with his mercy to extend forgiveness to such abominable provocation? What a man sows that shall he reap. If he sows to the flesh, he shall of the flesh reap corruption. Reader, it is a dreadful thing to fall into the hands of the living God.
3. Unbelief has generally been considered the most damning of all sins. I wish those who make this assertion would condescend to explain themselves. What is this unbelief that damns and ruins mankind? Their not permitting their minds to be persuaded of the truths which God speaks. Απιστια, from α, negative, and πιστις, faith, signifies faithless or to be without faith. And this is an effect from another cause. In Hebrews 4:11, these very people are said to have fallen through unbelief; but there the word is απειθεια, from α, negative, and πειθω, to persuade. They heard the Divine instructions, they saw God's stupendous miracles; but they would not suffer themselves to be persuaded, that he who said and did such things would perform those other things which he had either threatened or promised: hence they had no faith, because they were unpersuaded; and their unbelief was the effect of their unpersuaded or unpersuadable mind. And their minds were not persuaded of God's truth, because they had ears open only to the dictates of the flesh; see on Hebrews 4:2 (note). Here then is the damning sin, the not inferring, from what God has said and done, that he will do those other things which he has either threatened or promised. And how few are there who are not committing this sin daily! Reader, dost thou in this state dream of heaven? Awake out of sleep!
4. Where there are so many snares and dangers it is impossible to be too watchful and circumspect. Satan, as a roaring lion, as a subtle serpent, or in the guise of an angel of light, is momentarily going about seeking whom he may deceive, blind, and devour; and, when it is considered that the human heart, till entirely renewed, is on his side, it is a miracle of mercy that any soul escapes perdition: no man is safe any longer than he maintains the spirit of watchfulness and prayer; and to maintain such a spirit, he has need of all the means of grace. He who neglects any of them which the mercy of God has placed in his power, tempts the devil to tempt him. As a preventive of backsliding and apostasy, the apostle recommends mutual exhortation. No Christian should live for himself alone; he should consider his fellow Christian as a member of the same body, and feel for him accordingly, and have, succor, and protect him. When this is carefully attended to in religious society, Satan finds it very difficult to make an inroad on the Church; but when coldness, distance, and a want of brotherly love take place, Satan can attack each singly, and, by successive victories over individuals, soon make an easy conquest of the whole.

So we see that they could not enter in,.... To God's rest, the land of Canaan, for they died by the plague before the Lord, and their carcasses fell in the wilderness, before they came to it, Numbers 14:37 and the reason was,
because of unbelief; their distrust of God, his power, and his providence; this instance is produced by the apostle, to show the evil nature of unbelief, and the sad effects of it; to deter persons from it, and that they might take heed of encouraging it; and which instance he further improves in the following chapter.

they could not enter--though desiring it.

So we see they could not enter in - Though afterward they desired it.