
Dear Bishoy,
Most welcome! I am not the best person in the list to answer this question but as far as I know, yes, the term meant (and still means) "the chosen one." Comparing المجتبی with corresponding titles for the other evangelists, the case seems to be a result
of the impact of Muslim terminology on Christian literature, as Matthew was titled المصطفی and Luke المرتضی by some Christian authors. The case of the Evangelist John is somewhat different as he was referred to as الحبیب, among other titles, which comes from
John 20:2.