The important plural in your sentences is "
those." People often misuse "their" as a singular, and "who" may be either singular or plural.
If you wanted to express the same thought in the singular, you might use the singular "whoever" in place of the plural "those." "I feel sorry for whoever lacks faith in his life."
this gets messy because of the gender issue. People (me, for instance) don't like to use "his or her," so they replace these singular pronouns with the plural "their," and say, "I feel sorry for whoever lacks faith in their life." This is a real mess. Note that "whoever lacks" has a singular subject and a singular verb.
If a man were leading two lives, the possessive pronoun (his) would still have to be singular: "I feel sorry for the man who lacks faith in his lives," just like, "I feel sorry for the man who lacks strength in his legs." "Legs" is plural, but "his" must be singular because "man" is singular.
Best wishes, - A.
Edit. About the ambiguity, my question is, where exactly is the faith lacking? Do YOU lack faith, or does you LIFE lack faith? For example, you'd say that YOU lack courage, or your LIFE lacks excitement. But "faith" can work either way. "
I don't have enough faith in the way my life will turn out." "My
life doesn't seem to have enough faith in it." (faith, as an ingredient in my life)
Make any sense??