Here's the context of what you alluded to:
Hebrews chapter 3
[1] Wherefore, holy brethren, partakers of the heavenly calling, consider the Apostle and High Priest of our profession, Christ Jesus;
[2] Who was faithful to him that appointed him, as also Moses was faithful in all his house.
[3] For this man was counted worthy of more glory than Moses, inasmuch as he who hath builded the house hath more honour than the house.
[4] For every house is builded by some man; but he that built all things is God.
[5] And Moses verily was faithful in all his house, as a servant, for a testimony of those things which were to be spoken after;
[6] But Christ as a son over his own house; whose house are we, if we hold fast the confidence and the rejoicing of the hope firm unto the end.
[7] Wherefore (as the Holy Ghost saith, To day if ye will hear his voice,
[8] Harden not your hearts, as in the provocation, in the day of temptation in the wilderness:
[9] When your fathers tempted me, proved me, and saw my works forty years.
[10] Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways.
[11] So I sware in my wrath, They shall not enter into my rest.)
[12] Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God.
[13] But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin.
[14] For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end;
[15] While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation.
[16] For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
[17] But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
[18] And to whom sware he that they should not enter into his rest, but to them that believed not?
[19] So we see that they could not enter in because of unbelief.
Hebrews chapter 4
[1] Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it.
[2] For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.
[3] For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.
[4] For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.
[5] And in this place again, If they shall enter into my rest.
[6] Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:
[7] Again, he limiteth a certain day, saying in David, To day, after so long a time; as it is said, To day if ye will hear his voice, harden not your hearts.
[8] For if Jesus had given them rest, then would he not afterward have spoken of another day.
[9] There remaineth therefore a rest to the people of God.
[10] For he that is entered into his rest, he also hath ceased from his own works, as God did from his.
[11] Let us labour therefore to enter into that rest, lest any man fall after the same example of unbelief.
[12] For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart.
[13] Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do.
[14] Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.
[15] For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.
[16] Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.
Why do we need to come boldly unto the throne of grace that we may obtain mercy and find grace to help in time of need? Well, because WE CAN FALL AWAY on any given day that is called "Today" and we ought to therefore FEAR...even though you regularly foolishly and defiantly deny the same.
