“Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. For we are made partakers of Christ, if we hold the beginning of our confidence steadfast unto the end; While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation” (Hebrews 3:12-15).
From Matthew Henry’s Commentary about these verses: “But sin, long persisted in, will make God’s wrath discover itself in destroying the impenitent; there is no resting under the wrath of God. Take heed: all who would get safe to heaven must look about them; if once we allow ourselves to distrust God, we may soon desert him. Let those that think they stand, take heed lest they fall. Since to-morrow is not ours, we must make the best improvement of this day. And there are none, even the strongest of the flock, who do not need help of other Christians. Neither are there any so low and despised, but the care of their standing in the faith, and of their safety, belongs to all. Sin has so many ways and colours, that we need more eyes than ours own. Sin appears fair, but is vile; it appears pleasant, but is destructive; it promises much, but performs nothing. The deceitfulness of sin hardens the soul; one sin allowed makes way for another; and every act of sin confirms the habit. Let every one beware of sin.
And what is the unpardonable sin? “And to whom sware he that they should not enter into his rest, but to them that believed not? So we see that they could not enter in because of unbelief.” Unbelief.