Merriam-Webster.com – a bounding or limiting line especially one that when crossed commits a person irrevocably.
Thesaurus.com – point of no return
Romans 1:24, 28–32 (NIV84)
24Therefore God gave them over in the sinful desires of their hearts to sexual impurity for the degrading of their bodies with one another. . . .
28Furthermore, since they did not think it worthwhile to retain the knowledge of God, he gave them over to a depraved mind, to do what ought not to be done.29They have become filled with every kind of wickedness, evil, greed and depravity. They are full of envy, murder, strife, deceit and malice. They are gossips, 30slanderers, God-haters, insolent, arrogant and boastful; they invent ways of doing evil; they disobey their parents; 31they are senseless, faithless, heartless, ruthless. 32Although they know God’s righteous decree that those who do such things deserve death, they not only continue to do these very things but also approve of those who practice them.
2 Thessalonians 2:9–12 (NIV84)
9The coming of the lawless one will be in accordance with the work of Satan displayed in all kinds of counterfeit miracles, signs and wonders, 10and in every sort of evil that deceives those who are perishing. They perish because they refused to love the truth and so be saved. 11For this reason God sends them a powerful delusion so that they will believe the lie 12and so that all will be condemned who have not believed the truth but have delighted in wickedness.
There is a common misunderstanding that it’s never too late for a sinner to repent and be saved. There needs to be a caveat to that statement to make it true.
If one still has a desire to truly repent and be saved, it’s not too late.
However, the unredeemed need to know that there may come a time when their persistent rebellion and rejection may very well result in God sealing them in their unbelief. They will no longer have the capacity to desire salvation. Then it is too late.
The word “Rubicon” is not in the Bible, but the concept clearly is.