The meaning of this sentence is that when evil energy can gather and invade the human body, it means that the human body's righteous energy (resistance) is already in a weak state.

“Where evil gathers, its qi will be deficient” is a famous saying in the theory of traditional Chinese medicine. This is because the strength of righteous qi is one of the key factors that determine whether the human body is susceptible to evil.

When righteousness is sufficient, the body's immunity and resistance will be enhanced, making it less susceptible to evil. On the contrary, when righteousness is weak, the body's immunity and resistance will be reduced, making it less susceptible to evil.