"If we cultivate without striving for liberation,
then our cultivation in this life,
is in fact an enemy during our third rebirth."
In the first lifetime, the practitioner engages in mundane good deeds which bring ephemeral worldly blessings (wealth, power, authority, etc.) in the second lifetime.
Since power tends to corrupt, he is likely to create evil karma, resulting in retribution in the third lifetime.
Thus, good deeds in the first lifetime are potential "enemies" of the third lifetime.
In a mundane context, these three lifetimes can be conceived of as three generations.
Thus, the patriarch of a prominent family, through hard work and luck, amasses great power, fortune and influence (first lifetime).
His children are then able to enjoy a leisurely, and, too often, dissipated life (second lifetime).
By the generation of the grandchildren, the family's fortune and good reputation have all but disappeared (third lifetime).