What does it mean that a thousand-year-old monk once entered the world of mortals in the mountains?

Monks in the mountains have practiced for thousands of years, but once they enter the secular world of mortals, they will be infected with worldly troubles and disturbances.

The phrase "once a thousand-year-old monk in a mountain enters the world of mortals" means that once a monk in a mountain practices for thousands of years, once he enters the world of mortals, he will be infected with worldly troubles and disturbances. This sentence is usually used to describe the need for practitioners to maintain inner peace and purity in the process of practice and avoid being disturbed by secular materials and desires. Once entering the secular world of mortals, practitioners may be attracted by all kinds of temptations and interferences, thus losing the original intention and significance of practice.