The abstract noun "salvation" can be expressed with a phrase about God saving people. AT: "with fear and trembling, continue to work hard to do what is proper for those whom God saves" or "with awe and reverence for God, work hard to do the good things that show that he has saved you" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
Paul uses the words "fear" and "trembling" together to show the attitude of reverence that people should have for God. AT: "trembling with fear" or "with deep reverence" (See: [[rc://en/ta/man/translate/figs-doublet]])