Hebrews 2:3
how can we escape, if we pay no attention at all to a salvation that is so great? This is so because it was first proclaimed by the Lord Himself, and then it was proved to us to be valid by the very men who heard Him themselves,
Hebrews 6:6
and then have fallen by the wayside -- it is impossible, I say, to keep on restoring them to their first repentance, since they continue to crucify the Son of God to their detriment and hold Him up to contempt.
Hebrews 9:28
so Christ was offered once for all to take away the sins of many, but again He will appear, without having anything to do with sin, to save those who are eagerly waiting for Him to bring them final salvation.
Hebrews 10:26-29
For if we go willfully sinning after we have received full knowledge of the truth, there is no sacrifice left to be offered for our sins,
Hebrews 12:25
See to it that you do not refuse to listen to Him who is speaking to you. For if they did not escape, because they refused to listen to him who warned them here on earth, how much less can we, if we reject Him who is from heaven?