What makes the content abominable to God is the further description of the liquid: and [that is,] the impurities of her sexual immorality.
The term translated here as impurities
is the word commonly used in Scripture for unclean
. Something is unclean
because it in some way reminds of sin. This cup is loaded with abominable sin precisely because the transgression here depicted is one that denies the Godness of God, that is, that he alone is the source of happiness and comfort in life. By directing attention to created stuff as sources of happiness, God is robbed of what is his.
4 The woman was arrayed in purple and scarlet, and adorned with gold and jewels and pearls, holding in her hand a golden cup full of abominations and the impurities of her sexual immorality.