"The alien" is a symbol of a person whose physical life is transitory, fleeting, quickly "coming" and "going" from the person.

"The alien" is a symbol of the righteous man who explores, fulfills and evangelizes the Will of God, realizing the perishability, the temporality of his physical sojourn in the body.  

"The alien" is also a symbol of a person of a different worldview and way of life, different from the usual, common among most people.