"25See that ye refuse not him that speaketh. For if they escaped not who refused him that spake on earth, much more shall not we escape, if we turn away from him that speaketh from heaven: 26Whose voice then shook the earth: but now he hath promised, saying, Yet once more I shake not the earth only, but also heaven." (Hebrews 12: 25-26)
When the writer of Hebrews talks about "refusing", this warning is not about doing something right or wrong.
It's about believing the truth of the Gospel:
"How beautiful upon the mountains are the feet of him that bringeth good tidings, that publisheth peace; that bringeth good tidings of good, that publisheth salvation; that saith unto Zion, Thy God reigneth!" (Isaiah 52: 7)
"Who hath believed our report? and to whom is the arm of the LORD revealed?" (Isaiah 53:1)
The verse above starts out the chapter describing the sufferings of our Lord Jesus, who was crushed for our sins.
The Prophet Habakkuk revealed not just the Gospel, but the dangers of rejecting the truth of God's grace:
"Behold ye among the heathen, and regard, and wonder marvellously: for I will work a work in your days, which ye will not believe, though it be told you." (Habakkuk 1: 5)
"4Behold, his soul which is lifted up is not upright in him: but the just shall live by his faith." (Habakkuk 2: 4)
Paul later preached the Gospel:
"Be it known unto you therefore, men and brethren, that through this man is preached unto you the forgiveness of sins: 39And by him all that believe are justified from all things, from which ye could not be justified by the law of Moses" (Acts 12: 38-39)
. 40Beware therefore, lest that come upon you, which is spoken of in the prophets; 41Behold, ye despisers, and wonder, and perish: for I work a work in your days, a work which ye shall in no wise believe, though a man declare it unto you.:" (Acts 12: 40-41)
Because some of the Jews did not believe, Paul had no choice but to declare:
"46Then Paul and Barnabas waxed bold, and said, It was necessary that the word of God should first have been spoken to you: but seeing ye put it from you, and judge yourselves unworthy of everlasting life, lo, we turn to the Gentiles." (Acts 12: 46)
Do not be unbelieving, but believe the Gospel (Good News) today!