"Never before had Nero heard the truth as he heard it on this occasion. Never before had the enormous guilt of his own life been so revealed to him. The light of heaven pierced the sin-polluted chambers of his soul, and he trembled with terror at the thought of a tribunal before which he, the ruler of the world, would finally be arraigned, and his deeds receive their just award. He feared the apostle's God, and he dared not pass sentence upon Paul, against whom no accusation had been sustained. A sense of awe restrained for a time his bloodthirsty spirit. AA 496.1"
"And as he reasoned of righteousness, temperance, and judgment to come, Felix trembled, and answered, Go thy way for this time; when I have a convenient season, I will call for thee."
Nero's fear and trembling upon hearing the truth is similar to Felix's reaction to Paul's preaching about righteousness, temperance, and judgment.
"For a moment, heaven was opened to the guilty and hardened Nero, and its peace and purity seemed desirable. That moment the invitation of mercy was extended even to him. But only for a moment was the thought of pardon welcomed. Then the command was issued that Paul be taken back to his dungeon; and as the door closed upon the messenger of God, the door of repentance closed forever against the emperor of Rome. No ray of light from heaven was ever again to penetrate the darkness that enveloped him. Soon he was to suffer the retributive judgments of God. AA 496.2"
"For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame."
The description of Nero rejecting the invitation of mercy and the door of repentance closing against him parallels the warning against falling away after having been enlightened.