Both sides of this debate are off - the writer of Hebrews is referring to those who had tasted the witness the signs of Pentecost had been meant to be unto Israel (see Matt.10; Mark 16; Acts 2; Acts 3; Heb. 2; etc.) but that Israel had continued to reject said witness' intended result - Israel's repentance.
Hebrews 3:7 Wherefore (as the Holy Ghost saith, To day if ye will hear his voice, 3:8 Harden not your hearts, as in the provocation, in the day of temptation in the wilderness: 3:9 When your fathers tempted me, proved me, and saw my works forty years. 3:10 Wherefore I was grieved with that generation, and said, They do alway err in their heart; and they have not known my ways. 3:11 So I sware in my wrath, They shall not enter into my rest.) 3:12 Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God. 3:13 But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. 3:14 For we are made partakers of Christ, if we hold the beginning of our confidence stedfast unto the end; 3:15 While it is said, To day if ye will hear his voice, harden not your hearts, as in the provocation. 3:16 For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses. 3:17 But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness? 3:18 And to whom sware he that they should not enter into his rest, but to them that believed not? 3:19 So we see that they could not enter in because of unbelief.
Part of what throws throws off many is his use of the word "brethren."
But note how that is used, say, in Acts 3 (and elsewhere, when Israel is being addressed)...
Acts 3:14 But ye denied the Holy One and the Just, and desired a murderer to be granted unto you; 3:15 And killed the Prince of life, whom God hath raised from the dead; whereof we are witnesses. 3:16 And his name through faith in his name hath made this man strong, whom ye see and know: yea, the faith which is by him hath given him this perfect soundness in the presence of you all. 3:17 And now, brethren, I wot that through ignorance ye did it, as did also your rulers. 3:18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled. 3:19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord; 3:20 And he shall send Jesus Christ, which before was preached unto you: 3:21 Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
Another issue is that the writer of Hebrews is addressing things from the perspective of an Israelite.
Such often calls for including oneself in said addressing, as a member of that nation...
As in the case of one of the greatest examples of faith in all of Scripture - the Israelite Prophet: Daniel - repeatedly, he includes himself In his nation's NATIONAL guilt.
Daniel 9:2 In the first year of his reign I Daniel understood by books the number of the years, whereof the word of the LORD came to Jeremiah the prophet, that he would accomplish seventy years in the desolations of Jerusalem. 9:3 And I set my face unto the Lord God, to seek by prayer and supplications, with fasting, and sackcloth, and ashes: 9:4 And I prayed unto the LORD my God, and made my confession, and said, O Lord, the great and dreadful God, keeping the covenant and mercy to them that love him, and to them that keep his commandments; 9:5 We have sinned, and have committed iniquity, and have done wickedly, and have rebelled, even by departing from thy precepts and from thy judgments: 9:6 Neither have we hearkened unto thy servants the prophets, which spake in thy name to our kings, our princes, and our fathers, and to all the people of the land. 9:7 O LORD, righteousness belongeth unto thee, but unto us confusion of faces, as at this day; to the men of Judah, and to the inhabitants of Jerusalem, and unto all Israel, that are near, and that are far off, through all the countries whither thou hast driven them, because of their trespass that they have trespassed against thee. 9:8 O Lord, to us belongeth confusion of face, to our kings, to our princes, and to our fathers, because we have sinned against thee. 9:9 To the Lord our God belong mercies and forgivenesses, though we have rebelled against him; 9:10 Neither have we obeyed the voice of the LORD our God, to walk in his laws, which he set before us by his servants the prophets. 9:11 Yea, all Israel have transgressed thy law, even by departing, that they might not obey thy voice; therefore the curse is poured upon us, and the oath that is written in the law of Moses the servant of God, because we have sinned against him. 9:12 And he hath confirmed his words, which he spake against us, and against our judges that judged us, by bringing upon us a great evil: for under the whole heaven hath not been done as hath been done upon Jerusalem. 9:13 As it is written in the law of Moses, all this evil is come upon us: yet made we not our prayer before the LORD our God, that we might turn from our iniquities, and understand thy truth. 9:14 Therefore hath the LORD watched upon the evil, and brought it upon us: for the LORD our God is righteous in all his works which he doeth: for we obeyed not his voice. 9:15 And now, O Lord our God, that hast brought thy people forth out of the land of Egypt with a mighty hand, and hast gotten thee renown, as at this day; we have sinned, we have done wickedly. 9:16 O LORD, according to all thy righteousness, I beseech thee, let thine anger and thy fury be turned away from thy city Jerusalem, thy holy mountain: because for our sins, and for the iniquities of our fathers, Jerusalem and thy people are become a reproach to all that are about us. 9:17 Now therefore, O our God, hear the prayer of thy servant, and his supplications, and cause thy face to shine upon thy sanctuary that is desolate, for the Lord's sake. 9:18 O my God, incline thine ear, and hear; open thine eyes, and behold our desolations, and the city which is called by thy name: for we do not present our supplications before thee for our righteousnesses, but for thy great mercies. 9:19 O Lord, hear; O Lord, forgive; O Lord, hearken and do; defer not, for thine own sake, O my God: for thy city and thy people are called by thy name.
Also, o never mind, lol
Rom. 5:8
Acts 17:11,12
