"For the people who were in the land before you committed all these abominable acts, and the land thus became defiled" [Acharei Mos 18:27]
The Rema [R' Moshe Isserles] explained this verse by way of a parable: A father purchased an expensive and beautiful garment for his beloved son. The son, however, was not careful with the garment and he soiled it.
If the son had been careful not to sit in a dirty area, he could have defended himself before his father with the claim that the garment was so fine and delicate that it stained easily, and he should therefore not be held accountable.
But since the son sat in an area that was so muddy and full of dirt that even a coarse and inexpensive garment would have been stained, he has absolutely no way of excusing himself.
The same thing applies to the Jewish people, said the Rema. The Jewish nation is Hashem's precious and only son, blessed with souls so fine and delicate that even the slightest sin is capable of leaving a stain. They can therefore claim before Hashem that it is the soul's delicate nature that is responsible for its stain.
But if the Jewish people commit sins that are so severe that they are capable of contaminating even unrefined nations - as the verse states "For the people who were in the land before you committed all these abominable acts, and the land thus became defiled" - then they will have absolutely no way of defending themselves before their Father in Heaven.
Source: Rabbi Y. Bronstein