Ancient Babylon is alluded to in order to refer to Rome. The reason is evident, for Babylon and Rome share despicable qualities. Both were world-ruling cities filled with ungodliness, and a haughtiness which ignored impending doom. Thinking herself invincible, Babylon fell. The same would be true for Rome.
Rome did not stand alone in indecency, but produced a gravitational pull. The wealth and success of Rome attracted other powers, and her ungodliness spread – “For all nations have drunk of the wine of the wrath of her fornication” (v. 3). The inconvenient truth for Rome is a natural, and undying process – you reap what you sow (cf. Galatians 6:7-8). In response to her glorying in wealth and idolatry, committing spiritual fornication, she would be given sure and just punishment – “Therefore her plagues will come in one day – death and mourning and famine” (v. 8).
Another voice from heaven gave warning to Christians. In the midst of such persecution there is temptation to compromise and lose faith. To participate in drinking “the wine of the wrath of her fornication” would be to sign up for the same punishment. The Christian is always expected to remain sanctified, no matter the severity of the environment. To do so is to lay up heavenly treasures.